Add contributing instructions

[ejm714]

Supersedes #139 to create simpler contributing instructions in light of the incoming changes from #141 which will not require contributors to run make build to generate and commit the rendered markdown files.

Notable changes:

• Removes the need to file an issue and getting community up votes before making a change to the checklist given this process has not been followed previously and we have not seen any flame wars to date. Relevant discussions have been happening in the PRs themselves which seems sufficient.

If this is approved:

• remove the duplicative wiki page (https://github.com/drivendataorg/deon/wiki/Add-a-new-item-to-the-examples-table)
• add link on wiki home page with link to contributing instructions

[codecov]

Codecov Report

Merging #143 (ee3ee37) into main (3907956) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #143   +/-   ##
=======================================
  Coverage   96.80%   96.80%           
=======================================
  Files           6        6           
  Lines         188      188           
=======================================
  Hits          182      182           
  Misses          6        6           

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[glipstein]

LGTM! 🙌