contrib: Update instructions regarding Unicode updates

I've noticed we have instructions on how to update from newer Unicode
standard, but it didn't mention uname2c.h regeneration.

The following patch mentions that, also mentions that the Copyright years
of Unicode should be updated and adds a copy of NameAliases.txt which
is used for uname2c.h generation.

2023-03-16  Jakub Jelinek  <jakub@redhat.com>

	* unicode/README: Update to mention also makeuname2c.
	* unicode/NameAliases.txt: New file.