Added PR title prefixes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | not symfony related
| Fixed tickets | -

I think it is good to document the common PR title prefixes to ease the work for us. At first, I thought we should add this to the 'PR format', but I think using the old `[tag]` prefixes on PR titles is the best for this.

I'd like to hear your opinions about this! :microphone: