CONTRIBUTORS.rst needs clean-up

[maylee]

Suggested clean-ups for ​https://github.com/tahoe-lafs/tahoe-lafs/blob/master/CONTRIBUTORS.rst here

• add link to creating a good ticket (but do not explain inline - how/why)

• reduce the perceived importance of the towncrier stuff

• suggestion of how to deal with new contributors:

• They should communicate their intention to us first

• Encourage ppl to sign no backdoorsl.rst

• Talk about the issue before you put work into it - transfer of organizational knowledge

• New code must have appropriate tests written for it

Also, we should have only one contributor doc

• There also exists ​​​​​​​​​​​​​​.github/CONTRIBUTING.rst

• ​https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors