Add README.md with contributing instructions

R=chinmaygarde@google.com, chinmay@google.com

Review URL: https://codereview.chromium.org/1843803003 .

README.md

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+GURL
+====
+
+This repository contains a library for manipulating URLs in C++.
+
+# Contributing
+
+Please do not make pull requests against this repository!
+
+This repository uses [Mojo](https://github.com/domokit/mojo)'s code
+review and build bot infrastructure. Assuming you have
+[depot_tools](http://www.chromium.org/developers/how-tos/install-depot-tools)
+installed, then `git cl` commands, as described
+[here](https://github.com/domokit/mojo#contribute), should Just Work.