nlpsolver: This is considered internal code

Change-Id: Icd8566c2fce1a0ff3fb9471afd71858ec2f91bc5
Reviewed-on: https://gerrit.libreoffice.org/71982
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

nlpsolver/README

 This extension integrates into Calc and offers new Solver engines to use for optimizing nonlinear programming models.
+
+As there is no known upstream source for nlpsolver/ThirdParty/EvolutionarySolver,
+the code is considered internal and can be modified like any other internal code.
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