Adding note about the PdoSessionHandler BC break

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12833
| License       | MIT
| Doc PR        | symfony/symfony-docs#4557

Hi guys!

This adds an UPGRADE note about the PdoSessionHandler BC break in 2.6. I also added a section on top, which might be more controversial that highlights the BC breaks. That's because this document is long, and much of it is telling you about deprecated features. But what I first want to know is - what will break?

I'm also still interested in maybe adding the old PdoSessionHandler to 2.6 and allowing you to use that one to keep BC (still a BC break, but you could choose to use the old one). 

Thanks!