Update contributer guide to have a nice read.me

[Rich-T-kid]

Currently the contributor guide directory has an index.md file that explains what the other files contain as well as links to them. This would be better as a README.md file so that GitHub displays a nice file on the directory instead of having to go to index.md first.

Currently

Goal

[gabotechs]

These files are not supposed to be read from GitHub, these are just templates for generating the docs on https://datafusion-contrib.github.io/datafusion-distributed/ using Sphinx, and Sphinx ignores that readme.md.

If you find it confusing discovering how to access the contributor guide, it might be better to add a section in the main README.md that points to https://datafusion-contrib.github.io/datafusion-distributed/contributor-guide/index.html

[Rich-T-kid]

makes sense, did exactly that in #433