Add CONTRIBUTING.md

This was currently being done in README.md, but thats likely not
the best way to do this, CONTRIBUTING.md is shown to users
if they try to create PR on github.

So this CL pulls out the bits that point folks to Gerrit and puts
them in CONTRIBUTING.md

Change-Id: I1d2be0d89cbff72c0655ba341756dd0f54e2ea7f

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+---------------------------------
+Do not make GitHub pull requests!
+---------------------------------
+
+Ceres development happens on
+[Gerrit](https://ceres-solver.googlesource.com/), including both
+repository hosting and code reviews.
+
+This GitHub Repository is a continuously updated mirror which is
+primarily meant for issue tracking.
+
+Please see our
+[Contributing to Ceres Guide](http://ceres-solver.org/contributing.html)
+for more details.

README.md

@@ -1,3 +1,5 @@
+[![Build Status](https://travis-ci.org/ceres-solver/ceres-solver.svg?branch=master)](https://travis-ci.org/ceres-solver/ceres-solver)
+
 Ceres Solver
 ============
 
@@ -11,18 +13,3 @@ since 2010. Ceres Solver can solve two kinds of problems.
 
 Please see [ceres-solver.org](http://ceres-solver.org/) for more
 information.
-
-[![Build Status](https://travis-ci.org/ceres-solver/ceres-solver.svg?branch=master)](https://travis-ci.org/ceres-solver/ceres-solver)
-
-WARNING - Do not make GitHub pull requests!
--------------------------------------------
-
-Ceres development happens on
-[Gerrit](https://ceres-solver.googlesource.com/), including both
-repository hosting and code reviews. The GitHub Repository is a
-continuously updated mirror which is primarily meant for issue
-tracking. Please see our [Contributing to Ceres Guide](http://ceres-solver.org/contributing.html) for more details.
-
-The upstream Gerrit repository is
-
-    https://ceres-solver.googlesource.com/ceres-solver