Add RELEASING.md documentation.

[hdost]

While we try to de-centralize the release process we should establish consistent practice.

Relates #1326

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.0%. Comparing base (bcf1205) to head (78f6816).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1555   +/-   ##
=====================================
  Coverage   67.0%   67.0%           
=====================================
  Files        138     138           
  Lines      19515   19515           
=====================================
  Hits       13084   13084           
  Misses      6431    6431           

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