Add introduction to the setting system.
Fix spelling issues in architecture document.
Merge pull request #1173 from sbt/wip/more-architecture

Add introduction to the setting system
Add changes documentation for 0.13.1->0.13.2
Merge pull request #1174 from sbt/wip/release-notes-0.13.2

Add changes documentation for 0.13.1->0.13.2
When fragmenting Defaults, we mixed back settings in the wrong order.

* packageArtifacts is not cleared by defautlSettings
* Added a test for this behavior (this one test should ensure the ordering for most settings is correct.)

Fixes #1176
Merge pull request #1177 from sbt/wip/issue-1176

When fragmenting Defaults, we mixed back settings in the wrong order.
Allow end users to add Plugins, not just AutoPlugins

This allows plugins to define a Plugins instance that captures both the
plugin and its required dependencies.

Also fixed up some scaladocs that were wrong.
Merge pull request #1189 from jroper/project-add-plugins

Allow end users to add Plugins, not just AutoPlugins
Merging AutoPlugin work from "0.13.2" branch