state that snowflake_database var does not alter where change history table is created

[RichieBzzzt]

Minor alteration to the README to make it clear that using a two part name in the CHANGE_HISTORY_TABLE argument will result in the METADATA database being used to store the change history table, and not the value in the SNOWFLAKE_DATABASE argument.

[RichieBzzzt]

Happy to alter the wording on this. I assumed that the snowflake_database var would set the database being used in the context of the connection, as I'm used to SMO and ADO.NET, and probably other people used to those libraries would make the same assumption.