Update "Contributing" Section with New Dev On-Boarding

[oubiwann]

There was a recent email to the list that went into this; it should be documented:
https://groups.google.com/d/msg/lisp-flavoured-erlang/f1mF360pUC4/EOjokdzUDmQJ

Part of epic #98

[oubiwann]

Also, for docs contributors, these were recently posted to the mail list:

• bugs - https://github.com/lfe/docs/issues?q=is%3Aopen+is%3Aissue+label%3Abug
• enhancements - https://github.com/lfe/docs/issues?q=is%3Aopen+is%3Aissue+label%3Aenhancement
• features - https://github.com/lfe/docs/issues?q=is%3Aopen+is%3Aissue+label%3Afeature
• wishlist - https://github.com/lfe/docs/issues?q=is%3Aopen+is%3Aissue+label%3Awishlist