MAINT: add warning about nessai.gw

[mj-will]

Now that first release of nessai-gw is out, it's time to add a warning about the gw submodule.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.89%. Comparing base (8d19dc1) to head (1af637f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #457   +/-   ##
=======================================
  Coverage   93.89%   93.89%           
=======================================
  Files          72       72           
  Lines        7027     7029    +2     
=======================================
+ Hits         6598     6600    +2     
  Misses        429      429           

Flag	Coverage Δ
unittests	93.89% <100.00%> (+<0.01%)	⬆️

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