Add links to getcomposer.org.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3394 44740490-163a-0410-bde0-09ae8108e29a
Add instructions on how to migrate from an existing installation from subversion.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3395 44740490-163a-0410-bde0-09ae8108e29a