Centralize links to community forums

There are two places to learn about community for Jsonnet:

1. https://github.com/google/jsonnet/blob/master/README.md
2. http://jsonnet.org/contributing.html

(1) only lists the mailing list, and in passing and makes no mention of the IRC channel. (2) lists both. If one looks only on the GitHub repo, one might not realize that the IRC channel even exists.

I've also found that without providing instructions, users assume that one must have a Google account to join an email list on Google Groups (which is not the case), so I created this set of instructions for JanusGraph:

https://github.com/JanusGraph/janusgraph#community

Proposal to update the `README.md` file:

* add a Community section the README in the repo
* list both email list and IRC channel there
* explain how to join the mailing list without a Google account