Add contributing guidelines

[ccamara]

Create a contributing guidelines to help people help us. We might consider a link to add themselves as collaborators by adding a link to the @all-contributors bot usage documentation.

[hturner]

I like the use of all contributors!

[ccamara]

Contributing guidelines should have instructions on the fact that only source code will be updated to main branch, whereas rendered site will be automatically pushed to gh_pages branch via github actions (explanation here #2)