Add docs about new SSI render option

[weaverryan]

See symfony/symfony#10702

I haven't looked at any details whatsoever, but it looks like it's an alternative to ESI.

[nyroDev]

+1

[wouterj]

I think this should be done in book/http_cache. Some information can be found in http://symfony.com/blog/new-in-symfony-2-2-the-new-fragment-sub-framework and the PR referenced above

[RamosBruno]

Hi there,

Is it possible to add SSI doc ? Or I can follow what is made here to make SSI rendering https://github.com/KingCrunch/SSIBundle ?

Thanks guys :)

[javiereguiluz]

Closing in favor of #2129, which will document this and the other content render strategies.