Add docs: Migrating from 1.x to 2.x

This documents some of the known differences between pymssql 1.x and pymssql 2.x and things to be aware of when migrating, such as:

* Handling of ``uniqueidentifier`` columns
* Arguments to ``pymssql.connect``
* Parameter substitution

Let me know if I missed anything...

Cc: @rsyring, @damoxc, @ramiro, @kunxi, @sontek