add info about 'npm run c' to CONTRIBUTING.md

[jgravois]

our changelog parser zeros in commit message formatting that is generated by the command npm run c to categorize chores, bug fixes, features and breaking changes.

we need to advertise this to new contributors (and old grizzly bears like @tomwayson)

[tomwayson]

so just for clarity, for a PR like #131, if I had run npm run c on the first commit, but then gotten feedback in review that required changes that would affect the message, the right thing to do would have been a rebase and squash and pushed to the same PR, right?

[tomwayson]

Or an even better example: https://github.com/Esri/arcgis-rest-js/pull/115/commits - where other people have pulled the branch and made their own commits. In that case we still want to squash and push to get the right message, right?

[jgravois]

i think that the fact that the changelog parser ignores the extra commits means that its less important to squash. either way though.