Removing installation instructions

Hey guys!

We have the installation instructions in too many places. Ultimately, as we get the new symfony.com/download page and the downloader, this will reduce to just *one* spot (likely the download page, with a hand-off to continue by reading the docs).

On this repo, we should have no installation docs - it should just point to the central place. I also think we can just link to the current version of all of the docs for the bundle.

Thanks!