README: add better links, Markdown

Move the README to README.md so Gerrit can render it; currently
Gerrit only renders files named exactly "README.md" (for example at
https://go.googlesource.com/go).

Add more links to the README explaining how to file issues,
how to submit code changes, where to download the code to and
how to get it. Hopefully this should help people who go to
https://go.googlesource.com/time or https://github.com/golang/time
figure out how to get started with development.

Change-Id: I10f544c0bf2d17b49c5c9c9b096ac7f4b61c7cd2
Reviewed-on: https://go-review.googlesource.com/49891
Reviewed-by: Kevin Burke <[email protected]>

Author: kevinburke

README

@@ -0,0 +1,17 @@
+# Go Time
+
+This repository provides supplementary Go time packages.
+
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/time`. You can
+also manually git clone the repository to `$GOPATH/src/golang.org/x/time`.
+
+## Report Issues / Send Patches
+
+This repository uses Gerrit for code changes. To learn how to submit changes to
+this repository, see https://golang.org/doc/contribute.html.
+
+The main issue tracker for the time repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/time:" in the
+subject line, so it is easy to find.