PHPLIB-1707: Document BC policy

[paulinevos]

Closes PHPLIB-1707

Documents our BC policy, in that it refers to Symfony's which we also follow.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.72%. Comparing base (b353173) to head (297b71a).
⚠️ Report is 2 commits behind head on v2.x.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               v2.x    #1763   +/-   ##
=========================================
  Coverage     87.72%   87.72%           
  Complexity     3190     3190           
=========================================
  Files           424      424           
  Lines          6348     6348           
=========================================
  Hits           5569     5569           
  Misses          779      779           

Flag	Coverage Δ
6.0-replica_set	85.68% <ø> (ø)
6.0-server	81.69% <ø> (ø)
6.0-sharded_cluster	85.27% <ø> (ø)
8.0-replica_set	87.64% <ø> (ø)
8.0-server	82.57% <ø> (ø)
8.0-sharded_cluster	87.31% <ø> (ø)

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