write docs on how to build a new dbt adapter

[jthandy]

If someone wants to write a new adapter, they should have clearly documented guidelines on what needs to be done.

[jrandrews]

This would be helpful for me if I'm able to help write the Hive adapter in #559

[drewbanin]

@jrandrews that's great! I will put some time into building out these docs soon!

[drewbanin]

preliminary docs are here: https://docs.getdbt.com/docs/building-new-database-adapters

These aren't perfect, but going to leave this issue open so we can revisit with recent changes to dbt

[drewbanin]

This page is pretty far out of date. Let's update it as a part of this issue: https://docs.getdbt.com/v0.12/docs/building-new-database-adapters

[drewbanin]

New docs are here: https://docs.getdbt.com/docs/building-a-new-adapter

Closing this