Update readme with maintenence note

[kajoseph]

This PR updates the readme with an out of maintenance note with a pointer to the new app repo. It also makes some verbiage changes and updates refs to copay in package.json

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.27%. Comparing base (ec93459) to head (16e58e4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12182   +/-   ##
=======================================
  Coverage   28.27%   28.27%           
=======================================
  Files         246      246           
  Lines       22515    22515           
  Branches     3692     3692           
=======================================
  Hits         6366     6366           
  Misses      15569    15569           
  Partials      580      580           

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