Added details to README for contributing to the site

[alanwsmith]

I filled out the README in the community.hachyderm.io directory with some more details including how to fork and sync.

I also added links to a three Getting Started pages on the Hugo site that provide a nice overview.

[quintessence]

PR update (all PRs are getting this update): we're in the process of rolling out our CLA, which we're going to have all contributors sign moving forward. We're (I'm) going to circle back on the PRs once that's complete.

(Even the README, heh...)

[quintessence]

We have finally rolled out our new CLA \o/ The CLA is through the Nivenly Foundation (the non-profit we're spinning up to house Hachyderm and OSS projects). The Docs are being licensed as CC and code (for code work on Hachyderm, Aurae, etc.) is Apache 2.0. (I realize code work doesn't apply here; still need to sign both parts for now :) ). The CLA is signed via a CLA bot that integrates with our GitHub repo: basically all future PRs will validate that you've signed the CLA.

https://cla.nivenly.org/

Relevant note: your signature is private information and will not be shared publicly.

For this PR in particular: I made some rather significant changes to the README file - could you resolve with your changes here? Thank you!