Support plan for cassandra driver 3.x

[michaelmcfadyensky]

As we are upgrading the cassandra driver used by cqlmigrate from 3.x to 4.x, we should consider if/how we want to support the users of cqlmigrate that cannot currently use the 4.x version of the cassandra driver and provide documentation on what our support model will look like.

Some questions to consider:

• Do we want to maintain support for a 3.x cassandra driver compatible version of cqlmigrate?
• What would be our policy for back porting new features to this?
• What would be our policy for back porting bug fixes to this?
• For how long would we want to support this for?