Add Contributing language

[pcothenet]

What

• Add a paragraph of guidelines for contributing

Why

• Tell potential contributors that building new features directly might not be the most efficient use of their time

SDK Release Checklist

• Have you added an integration test for the changes?
• Have you built the package locally and made queries against it successfully?
• Did you update the changelog?
• Did you bump the package version?
• For breaking changes, did you plan for the release of the new SDK versions and deploy the API to production?

[pcothenet]

@alianam @kleinjm putting something tentatively. Let me know what you think!

[alianam]

Nice!