created more detailed contributing.md

[IshaShah27]

#89

Used old instructions from the readme, a quick chat with Emily, and Peter's guide to adding to example tables to create CONTRIBUTING.md:

• instructions on how to add an item to the checklist
• instructions on how to add an example (taken from Peter's entry in the wiki)
• instructions how to prepare submissions (make build)
• instructions on what to submit in a PR
• review content of CONTRIBUTING.md - instructions and conventions (e.g., the part where it says an addition to the checklist must have 10 upvotes from community)
• remove redundant/outdated material from README.md
• link properly to CONTRIBUTING.md from the "Changing the checklist" section and the "Where things have gone wrong" section

[codecov]

Codecov Report

Merging #139 (4a4f05b) into main (0ad8fec) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #139   +/-   ##
=======================================
  Coverage   96.80%   96.80%           
=======================================
  Files           6        6           
  Lines         188      188           
=======================================
  Hits          182      182           
  Misses          6        6           

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[ejm714]

Thanks for this draft @IshaShah27. I've integrated and simplified this in #143