Some fixes and installation documentation

[MarceauKa]

Hi guys! I was here to write some doc wheb I have issues trying to install OpnForm locally.

So, you'll find:

• Documentation for the installation
• Some fixes in a migration: useless DB Transaction (not so useless after reading the process, but don't work with MariaDB here), and fixes malformed foreign keys constraints
• .gitignore for the storage/clockwork folder