Add contributing guide

Thanks https://github.com/pengwynn/flint!

Before:

```
$ ./flint
[WARNING] Contributing guide not found
[FIXME] Add a CONTRIBUTING guide for potential contributors. http://git.io/z-TiGg
[WARNING] Some warnings found.
```

After:

```
$ ./flint
[OK] All is well!
```

/cc @pengwynn