Add migration Instructions to Wiki

[derflocki]

I'm currently updating one of my projects to this version of friendsofsymfony1/symfony1. Currently everything went pretty easy, so thanks for your great work! My plan is to migrate ~10 more legacy projects, with significantly more code.

If you want, I would start updating the Wiki with install and upgrade instructions.

[connorhu]

It would be great if you could write a proper summary of the migration process. That could close issues #260 and #244

[thirsch]

Sounds great, thank you! Maybe updating https://github.com/FriendsOfSymfony1/symfony1/blob/master/UPGRADE.md in a pr with any advices and hints?

[connorhu]

One of my project needs to be migrated, I will do this issue.

[connorhu]

First draft
https://github.com/FriendsOfSymfony1/symfony1/wiki/Migrations

[alquerci]

@connorhu that's a good start.