Clean up CONTRIBUTING

[beaurancourt]

refs https://github.com/keep-network/pm/issues/3

This is part of a broader cleanup effort to add/update all of our main repos with a CONTRIBUTING file. In the process, I

• Standardize line length to 80
• Add, update, and verify installation instructions for each repo

tagging @pdyraga for review

[dimpar]

It looks like this repo is for tBTCv1 which we are sunsetting. I don't think we need to update anything here now.

[beaurancourt]

works for me!