Added CONTRIBUTING.md

[wbrgss]

Follow-up on a task in the onboarding checklist:

I have access to the Github repos I need to do my work and I've read their CONTRIBUTING docs or complained that they don't exist

The advantage of having a CONTRIBUTING.md file is that GitHub will suggest reading it when contributors open their first issue or PR:

I'm not intimately familiar with the workflow on this repo, so this should be considered a first stab at a more helpful guide, especially for open source contributors. Related: PR #243 on dash-core-components

[cldougl]

@wbrgss awesome much-needed addition!!
no blocking issues for me so
💃after considering my comments 🎉

[cldougl]

let's change:

Using the [GitHub flow][] is encouraged

to
Use the [GitHub flow][]

I'd say this is more mandatory than encouraged 😆

[cldougl]

then grammatically I think we'd want to change the last bit to:
(i.e. create a feature branch and submit a PR against the master branch).

[cldougl]

Not blocking-- but I would suggest removing the note and just working the link https://dev.w3.org/html5/html-author/#index-of-elements inline in the contribution section. That's a good comprehensive list so I find it important enough to be inline and we may consider adding a notes section in the future when we have more notes to add.