Add github URL in README

So that imports into other projects (e.g. git) link to the upstream
repository.

Author: sthibaul

README.md

@@ -322,6 +322,10 @@ void test_example__a_test_with_auxiliary_methods(void)
 About Clar
 ==========
 
+Clar is currently maintained on [github][github].
+
+[github]: https://github.com/clar-test/clar
+
 Clar was originally written by [Vicent Martí](https://github.com/vmg),
 to replace the old testing framework in [libgit2][libgit2]. It is
 currently maintained by [Edward Thomson](https://github.com/ethomson),