First cut at a 0.13.2 migration guide.

* Covers how to migrate to autoplugins
* Covers breaking semantic changes in 0.13.2
* Covers basics of incremental tasks + .previous
* Not wired into the doc index yet.
* Does not cover name-hashing.

Review by @havocp or @cunei 
Additional review by @gkossakowski (including help with any incremental compiler relevant sections)