From a TODO in #1211. Add documentation for contributors.

[adrian-rivera]

This changes the README file and adds a new section Contributors.
This section includes instructions to remind contributors to make an
update to the changelog.

This relates to #1211

[keith-hall]

Thanks for your contribution.
As per the comment at #1211 (comment), I think we ideally want this info in a separate CONTRIBUTING.md file, which could be linked to from the readme.

[eth-p]

I feel like the file should be renamed. Although there isn't really any standard for this, I think CONTRIBUTING.MD is used more often than CONTRIBUTORS.MD.

For example, this is what Hacktoberfest has to say:

Do: Follow a repository's contribution guidelines

Almost every repo looking for help has a CONTRIBUTING.md file in it, read it first!

[adrian-rivera]

I feel like the file should be renamed. Although there isn't really any standard for this, I think CONTRIBUTING.MD is used more often than CONTRIBUTORS.MD.

For example, this is what Hacktoberfest has to say:

Do: Follow a repository's contribution guidelines

Almost every repo looking for help has a CONTRIBUTING.md file in it, read it first!

Yep, I think you are right, CONTRIBUTING seems to be widely use, I'll do the change

[sharkdp]

Thank you for starting this @adrian-rivera. I added a few more sections to the CONTRIBUTING.md file.

@eth-p @keith-hall Let me know if you think we should add anything else to the document.

[eth-p]

Looks good to me!