[core] create CONTRIBUTING file

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+Ways to Contribute
+=================
+
+We appreciate contributions from the community! Some of the ways to contribute are:
++ Bug fixes, whether they be for performance or correctness
++ Support for additional input formats
++ Additional kernel implementations
+
+Although the GAP Benchmark Suite only specifies 6 graph kernels, we are open to adding more non-benchmark kernels to this repo. It is our goal to provide a set of portable high-performance baselines, so naturally there are plenty of remaining graph problems (e.g. MST, SCC, ALS, etc...).
+
+For code contributions, please focus on code simplicity and readability. This repo follows nearly all of the [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html). If you open a pull request, our continuous-integration service run by Travis will do an immediate sanity-check.
+
+Before embarking on a large development task, feel to open an issue or email (sbeamer@lbl.gov) to ensure you are on a profitable path.