Attempt to clarify the note on postgres support

[jpcamara]

• On twitter, some folks (myself included) were confused about the note and thought it was implying some issue with using this gem with Postgres

• This tries to emphasize first that all three are fully supported. Then it tries to specify a bit more clearly the distinction about the action cable built-in adapter vs solid cable

• Also mentions potentially hitting issues with the payload size, or not wanting the NOTIFY command (which may be a problem for some folks using pgbouncer)

[jpcamara]

Good call @npezza93 - I just updated it to point at the rails docs instead