Removing installation instructions

We have this multiple other places
minor #751 Removing installation instructions (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
----------

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!

Commits
-------

22589ce Removing installation instructions
changed version to 2.3 in the README
Merge branch '2.3' into 2.5

* 2.3:
  changed version to 2.3 in the README
  Removing installation instructions

Conflicts:
	README.md
Merge branch '2.5' into 2.6

* 2.5:
  changed version to 2.3 in the README
  Removing installation instructions

Conflicts:
	README.md
Merge branch '2.6' into 2.7

* 2.6:
  changed version to 2.3 in the README
  Removing installation instructions
updated VERSION in README