Replace some .md files with .org files

Although not required, this is motivated by issue greghendershott#389. A "README.org"
will be automatically recognized without the package definition
needing a :readme item. Furthermore, this is an Emacs package, we were
already using org instead of markdown for some other files, and we may
as well be consistent.

CONTRIBUTING.org

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+* Reporting bugs
+
+If you're going to report a bug --- thank you!
+
+Please use =M-x racket-bug-report= to generate a buffer with
+information that will help to reproduce and understand the bug:
+
+- Emacs version.
+- Value of important Racket Mode variables.
+- Minor modes that are active.
+
+Please copy that and paste in your bug report.
+
+* Making pull requests
+
+If you'd like to make a pull request --- thank you!
+
+Here is some information to help you.
+
+** Package dependencies
+
+For end users, Racket Mode currently has zero dependencies on other
+packages --- in =racket-mode.el= =Package-Requires:= is just:
+
+#+BEGIN_SRC elisp
+;; Package-Requires: ((emacs "25.1"))
+#+END_SRC
+
+For hacking on Racket Mode and to run tests, a couple packages are
+required. To install them: =make deps=.
+
+The recent trend has been for Racket Mode to depend on fewer packages,
+not more. For example =dash.el= and =s.el= were dropped in favor of
+directly using the built-in Emacs functions wrapped by those packages.
+
+Having said that, if your PR proposes adding a dependency on a new
+package that you think is worthwhile, please make sure your PR updates
+both:
+
+1. the =Package-Requires:= line in =racket-mode.el=
+2. the =deps= target in =Makefile=
+
+** Pointing Emacs to your Git clone
+
+After installing dependencies you should just need to add the path to
+your local clone of Racket Mode to =load-path= and require it:
+
+#+BEGIN_SRC elisp
+(add-to-list 'load-path "/path/to/the/git-clone/dir")
+(require 'racket-mode)
+#+END_SRC
+
+If you use =use-package=, you can simply replace
+
+#+BEGIN_SRC elisp
+(use-package racket-mode
+  :ensure t)
+#+END_SRC
+
+with
+
+#+BEGIN_SRC elisp
+(use-package racket-mode
+  :load-path "/path/to/the/git-clone/dir")
+#+END_SRC
+
+If you have previously been using Racket Mode as a package installed
+from MELPA, you might want to remove that, at least for the duration
+of your hacking:
+
+- =M-x package-delete= and enter =racket-mode=.
+- Restart Emacs.
+
+** Generating reference documentation
+
+We generate reference documentation from doc strings for commands,
+variables, and faces.
+
+- If you add a brand-new command =defun=, =defcustom=, or =defface=,
+  please also add it to the appropriate list in =doc/generate.el=.
+
+- Whenever you edit a doc string for a command =defun=, =defcustom=,
+  or =defface=, please =cd doc && make clean docs=, and commit the
+  updated files.
+
+** Tests
+
+Currently tests are on the light side. More are welcome.
+
+Please do run =make test= locally to ensure your changes pass the
+existing tests.
+
+GitHub Actions also does =make test= automatically on your pull
+request.
+
+GitHub branch protection is enabled for the main branch --- merges
+are blocked until tests pass.
+
+*** Example files for indentation and font-lock
+
+Some Racket Mode tests apply indentation and font-lock to the
+=test/example/example.rkt= and =test/example/indent.rkt= files and
+compare the result to corresponding =.faceup= files (generated by the
+=faceup= package).
+
+As a result, if your PR intentionally modifies indentation or
+font-lock, you may need to regenerate the =.faceup= files. To do so:
+
+1. Disable any personal Emacs features that affect font-lock or
+   indentation. For example you may need to =M-x global-paren-mode=
+   and =M-x prettify-symbols-mode= to disable those.
+
+2. For each =.rkt= file:
+
+    - Visit the =.rkt= file.
+
+    - =M-x mark-buffer= and =M-x indent-region=.
+
+    - =M-x save-buffer= to save the =.rkt= file.
+
+    - =M-x faceup-write-file= and answer, yes, replace the existing
+      =.faceup= file.
+
+3. Re-enable any personal features you disabled in step 1.

README.org

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+* Racket mode for GNU Emacs
+
+[[https://github.com/greghendershott/racket-mode/actions][https://github.com/greghendershott/racket-mode/workflows/CI/badge.svg]]
+[[https://melpa.org/#/racket-mode][https://melpa.org/packages/racket-mode-badge.svg]]
+[[https://www.racket-mode.com/][https://img.shields.io/badge/Docs-Documentation-blue.svg]]
+
+This provides a major mode to edit [[https://www.racket-lang.org/][Racket]] source files, as well as a
+major mode for a Racket REPL. The edit/run experience is similar to
+[[https://docs.racket-lang.org/drracket/][DrRacket]].
+
+Compatible with *Emacs 25.1+* and *Racket 6.9+*.
+
+** Documentation
+
+See the [[https://www.racket-mode.com/][Guide and Reference]].
+
+** Contributing
+
+Pull requests are welcome; please [[file:CONTRIBUTING.org][read this]].
+
+[[file:THANKS.org][Acknowledgments]].
+
+** Alternatives
+
+- Emacs' built-in `scheme-mode` major mode plus the minor modes  [[https://www.neilvandyke.org/quack/][Quack]]
+  and/or [[https://www.nongnu.org/geiser/][Geiser]].

THANKS.org

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+* Contributors
+
+Thanks to everyone who has contributed [[https://github.com/greghendershott/racket-mode/graphs/contributors][pull requests]] and [[https://github.com/greghendershott/racket-mode/issues?utf8%3D%25E2%259C%2593&q%3Dis%253Aissue][issues]].
+
+** Acknowledgements
+
+- The existing Emacs Scheme mode and Inferior Scheme mode.
+
+- The source code for for [[http://www.neilvandyke.org/quack/][Quack]] by Neil Van Dyke provided a model for
+  many of the scheme-indent-function settings, smart paren closing,
+  and pretty lambda.
+
+- The source code for [[http://www.nongnu.org/geiser/][Geiser]] by Jose A. Ortega Ruiz helped me
+  understand how to support completions and especially company-mode.
+  In addition, I was able to make heavy use of a pull request to
+  display images in the REPL.