Make README indicate that GitHub is only a mirror

Change-Id: I1924021e676f6b007f06548a95410c802f506176

README

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-Ceres Solver - A non-linear least squares minimizer
-==================================================
-Please see docs/html/index.html for a tutorial and reference.

README.rst

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+Ceres Solver - A non-linear least squares minimizer
+===================================================
+Please see `ceres-solver.org <http://ceres-solver.org/>`_ for more information.
+
+WARNING - Do not make GitHub pull requests!
+-------------------------------------------
+Ceres development happens on Gerrit, 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 guide
+<http://ceres-solver.org/contributing.html>`_ for more details.
+
+The upstream Gerrit repository is::
+
+  https://ceres-solver.googlesource.com/ceres-solver