update readme and add a contributing.md file

[C-Loftus]

The previous readme did not specify that cargo run, cargo fmt and cargo clippy should be passing locally before committing. This may be obvious to those who are experienced in the Rust ecosystem, but for those who are not it is useful to specify and makes it easier for new contributors to quickly get up to speed.

I moved the information regarding flame graph as well into the contributing file, since it is likely not relevant for most users unless they are contributing. Finally, I chose to choose the name CONTRIBUTING.md instead of HACKING.md since CONTRIBUTING.md is more standardized, and has a specific special icon support in visual studio code among other support. (I also feel like long term, having a file called HACKING can be scary for non-technical users, even though that isn't particularly relevant at the moment.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 14.30%. Comparing base (026b7f4) to head (c3abecd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   14.30%   14.30%           
=======================================
  Files          17       17           
  Lines        1559     1559           
=======================================
  Hits          223      223           
  Misses       1336     1336           

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[TTWNO]

Thanks for putting this together! LGTM!