Add some tips for running Vagrant in Common Issues section
Update docs with `included` addon hook

Includes a brief demonstration of using `included` to inject your
own preprocessor into the app or parentAddon.
Move upgrading info to it's own section
Add outputPaths explanation and documentation #1888
Merge pull request #2509 from peterchoo/gh-pages

Adjust podModulePrefix example to be less confusing
Merge pull request #2477 from tstirrat/outputPaths-docs

[DOCS] Output path configuration documentation
Merge pull request #2427 from HeroicEric/gh-pages

[DOCS] Move upgrading info to it's own section
Merge pull request #2382 from davewasmer/gh-pages

Update docs to mention `included` addon hook
Merge pull request #2380 from zackangelo/docs-vagrant-tips

Add some tips for running Vagrant in Common Issues section
Clarify using Brocfile.js in an addon