Update docs to include other databases supported through the postgres connector

[hughess]

@joshnuss mentioned in #95 that Clickhouse is accessible through the postgres connector. There are other databases that can be accessed through the postgres connector, so I'm opening this issue to keep track of them so we can add them to the docs.

So far, these databases come to mind:

• Clickhouse
• TimescaleDB
• Redshift

If anyone has any other examples, please comment here and we'll add them to the docs! Also let us know if there are any special considerations for connecting to each one.