Update and rename CONTRIBUTING to CONTRIBUTING.md (#225)

CONTRIBUTING → CONTRIBUTING.md

 Want to contribute? Great! First, read this page (including the small print at the end).
 
 ### Before you contribute
+
 Before we can use your code, you must sign the
-[Google Individual Contributor License Agreement]
-(https://cla.developers.google.com/about/google-individual)
+[Google Individual Contributor License Agreement](https://cla.developers.google.com/about/google-individual)
 (CLA), which you can do online. The CLA is necessary mainly because you own the
 copyright to your changes, even after your contribution becomes part of our
 codebase, so we need your permission to use and distribute your code. We also
 frustration later on.
 
 ### Code reviews
-All submissions, including submissions by project members, require review. We
-use Github pull requests for this purpose.
+
+All submissions, including submissions by project members, require review.  
+We use Github pull requests for this purpose.
 
 ### The small print
-Contributions made by corporations are covered by a different agreement than
-the one above, the
-[Software Grant and Corporate Contributor License Agreement]
-(https://cla.developers.google.com/about/google-corporate).
+
+Contributions made by corporations are covered by a different agreement than the one above,  
+the [Software Grant and Corporate Contributor License Agreement](https://cla.developers.google.com/about/google-corporate).