Fix #1 - Migrate challenges from Slack to GitHub

Challenge text moved to markdown README files with a subdirectory
for each challenge.

Also added:
- guidelines for contributing in CONTRIBUTE.md
- 'sowpods.txt' sample lexicon file for scrabble words challenge

Author: imattman

00-sorting/README.md

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+# Warmup Challenge: Sorting
+
+Given a list of strings where each string is 2 words, a _first name_ and _last name_ separated by a space, write a function that sorts the list by last name, *then* first name.
+
+> example:
+```python
+["Anna Smith", "John Anderson", "Cliff Bates"]
+```
+> sorts to:
+```python
+["John Anderson", "Cliff Bates", "Anna Smith"]
+```

00-sorting/solutions/imattamn/README.md

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+# WIP

01-matrix-rotate/README.md

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+# Challenge: Matrix Rotation
+
+Given an _N x N_ matrix represented as a list of lists, write a function that rotates the matrix by 90 degrees clockwise.
+
+> examples:
+```python
+[[1, 2],           -- rotate -->  [[3, 1],
+ [3, 4]]                           [4, 2]]
+
+
+[['a', 'b', 'c'],                 [['x', 1, 'a'],
+ [ 1,   2,   3 ],  -- rotate -->   ['y', 2, 'b'],
+ ['x', 'y', 'z']]                  ['z', 3, 'c']]
+```
+
+Try writing two implementations: with and without using `zip()`.

02-caesar-cipher/README.md

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+# Challenge: Caesar Cipher
+
+Given two arguments `message` (string) and `shift` (int), write a function that enciphers the message using a caesar (substitution) cipher with the supplied shift value.
+
+Numbers, punctuation, and other non-alphabet characters should be passed through unchanged.
+Letter case should be preserved.
+
+> examples:
+```python
+message = "ABC 123 xyz"
+print(caesar(message, 2))  # prints "CDE 123 zab"
+
+message = 'Myxqbkdevkdsyxc, iye mbkmuon dro myno'
+print(caesar(message, 16))  # prints ???
+```

03-scrabble-words/README.md

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+# Challenge:  Scrabble Word Finder
+
+Write a program that reads in a lexicon file of valid dictionary words.  The program then accepts a player's rack of letters (a list size 1-10) and produces the top _N_ playable words based on scrabble score.
+
+A word is considered playable only if it is contained in the lexicon and all letters required to form the word are in the supplied rack.
+
+See [sowpods.txt](./sowpods.txt) as an example lexicon of valid words.