Updated README.md to remove Arm64 support

[LMRW]

Arm64 support has been broken for months.

README.md has been updated to reflect this.

This is the honourable thing to do, otherwise users will waste their time assuming it is supported, when it is not.

Many users are wasting hours of time assuming they are at fault, so it is correct to update README.md to let them know to not waste their time.

See https://github.com/CollaboraOnline/richdocumentscode/issues/226

[amplifydpete]

Well done!

[petergilmour1987]

Thank you!