Document 1.2 changes and migrations

[withinboredom]

Describe the feature

SDK version 1.2 includes a new dapr client and begins the deprecation of few classes. The changes need to be documented in dapr docs site and in the readme.

Migration from 1.1 to 1.2 needs to be documented, though it is pretty straightforward.

Release Note

RELEASE NOTE: ADD documentation for 1.2