Add concrete5 as a member project

Adding an entry as we'd like it to appear since [we've reached quorum](https://groups.google.com/forum/#!msg/php-fig/OI2I8xPz1Qs/0_RZqYfFsikJ) pending official final count.
Merge pull request #122 from KorvinSzanto/patch-1

Add concrete5 as a member project