Modify "conventions" to document how to contribute & the mailing list
Modify "conventions" to document how to contribute & the mailing list

  Modify the "conventions" section to point out how to contribute
  (submit a pull request against the "develop" branch) and the
  mailing list.  In some sense these are different than other
  conventions, because they're technology-specific - no doubt they'll
  change over time.  But other conventions will no doubt change over time
  as well, and we want to make sure that readers of this will have
  information on these things.  Putting this information here will
  hopefully help on-ramp new potential contributors, by letting them
  know where the repo and mailing list (group) are.

metamath/set.mm,metamath/set.mm,metamath/set.mm