Document how to use clang-format and add a test

Author: sebastianpoeplau

CONTRIBUTING.md

@@ -2,8 +2,12 @@
 
 We encourage everyone to contribute improvements and bug fixes to SymCC. Our
 preferred way of accepting contributions is via GitHub pull requests. Please be
-sure to run clang-format on any C/C++ code you change, and ideally add a test to
-your patch. Unfortunately, since the project is a bit short on developers at the
+sure to run clang-format on any C/C++ code you change; an easy way to do so is
+with `git clang-format --style LLVM` just before committing. (On Ubuntu, you can
+get `git-clang-format` via `apt install clang-format`.) Ideally, also add a test
+to your patch (see the
+[docs](https://github.com/eurecom-s3/symcc/blob/master/docs/Testing.txt) for
+details). Unfortunately, since the project is a bit short on developers at the
 moment, we have to ask for your patience while we review your PR.
 
 Please note that any contributions you make are licensed under the same terms as