Remove ideas for improvement (they're complete or near completion) and mention Flynn.
Pull the advanced installation sections into their own document
Update environment variable section to reflect new config plugin
Add upgrade instructions for users pre-27d4bc8c3c
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Merge pull request #303 from kkurian/patch-1

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dotcloud/docker#1869 is closed and released
Merge pull request #307 from mmalecki/patch-1

dotcloud/docker#1869 is closed and released
Add an 'add-ons' plugin, which provides heroku style add-ons.

Add-ons provide access to services (e.g. database) by applications.
This is still WIP.
Add an ADDONS.md file to describe how to use the plugin.
Fix indentation, refactor app/addon checking and add add-on private directory.
Add a help message to the add-on plugin.
Small fixes to add-ons plugin.

Sort add-ons list, add missing cat at the end of commands and remove
trailing whitespaces in pre-release.
Unprovision all add-ons in 'pre-delete'.
Fix a bug where the add-ons plugin would delete all ENV file.
Add enable/disable to the add-on plugin.

Add-ons must be enabled before they are used. This allows add-ons to
install dependencies, allocate necessary resources, ...
export ADDON_ROOT in the pre-release hook
Refactorize a lot of code, and rename ADDON_ROOT to ADDON_DATA.