CPack/NSIS: Document BOM for licence encoded in UTF-8

The only hint that the file needs to have a BOM mark is included in release notes:

• https://cmake.org/cmake/help/latest/release/3.20.html#cpack

It would make a lot more sense to document this next to the option name. (I first wanted to file a feature request to support UTF-8.)

Backport: release