Add docs for updating cassettes

[korikuzma]

In #83 I had to update cassettes (was getting a vcr.errors.CannotOverwriteExistingCassetteException: Can't overwrite existing cassette ('/home/runner/work/AnyVLM/AnyVLM/tests/unit/anyvar/cassettes/test_clients/test_put_allele_expressions_handle_invalid[anyvar_populated_python_client].yaml') in your current record mode ('none').). I don't believe documentation exists for this (and only new what to do due to experience with VRS-Python). It would be nice to have docs on how to setup / connect to seqrepo-rest instance / etc

[jsstevenson]

This will be for @theferrit32 or @korikuzma -- @jsstevenson is excused