Symlink the CONTRIBUTING file to the root of the project.

- Add markdown styling.

CONTRIBUTING.txt

@@ -0,0 +1 @@
+opensource/CONTRIBUTING.txt

opensource/CONTRIBUTING.txt

@@ -1,6 +1,9 @@
+# Contributing
+
 Want to contribute? Great! First, read this page (including the small print at the end).
 
-### Before you contribute
+## 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)
@@ -16,11 +19,13 @@ us first through the issue tracker with your idea so that we can help out and
 possibly guide you. Coordinating up front makes it much easier to avoid
 frustration later on.
 
-### Code reviews
+## Code reviews
+
 All submissions, including submissions by project members, require review. We
 use Github pull requests for this purpose.
 
-### The small print
+## 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]