[WIP][Cookbook] Document how to install 3th party bundles

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.1+
| Fixed tickets | #2176

Based ob this answer by Wouter J: http://stackoverflow.com/questions/13338692/how-do-you-register-bundles-in-symfony2-1-designed-for-older-versions-of-symfony/13340180#13340180

I used FOSUserBundle for an example as it is wider known.

For config:dump-reference command i used AsseticBundle because FOSUserBundle requires more effort to start working (mainly creating model classes). And it will be easier for new users to test with AsseticBundle.