Merge the two contributing guides into one

There are currently two contributing guides:

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

and the first one refers users to the second one (but it's a plain text file so the link is not even clickable). For simplicity and to improve usability, I recommend merging them into a single one, and that one should be in the code tree, because that is where the potential contributors will expect to find it.

A sample contributing file with plenty of details can be found in [this repo](https://github.com/mbrukman/autogen/blob/master/CONTRIBUTING.md). Thoughts / objections?