github: add PR template

[dfawley]

Hopefully including a PR template will improve the quality of the PR process.

The following might(?) provide a preview of the template in action: https://github.com/dfawley/grpc-go/compare/master...dfawley:grpc-go:test?expand=1, assuming github lets you create a PR from one of my branches to my repo. Otherwise you can copy/paste the text of the file into a blank PR anywhere to see the markdown preview. Users will not see the preview unless they select the appropriate tab, so it needs to be formatted well as plain-text, too.

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.95%. Comparing base (fa0d658) to head (a46aa62).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8524      +/-   ##
==========================================
+ Coverage   81.86%   81.95%   +0.08%     
==========================================
  Files         412      412              
  Lines       40518    40518              
==========================================
+ Hits        33172    33207      +35     
+ Misses       5953     5924      -29     
+ Partials     1393     1387       -6     

see 15 files with indirect coverage changes

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