Adding an official format to be used for documentation pull requests

Hi guys!

This introduces a new PR format for the documentation, similar to what was
just introduced for the core repo and as recommended by @WouterJ in our ongoing
conversation at https://groups.google.com/forum/#!msg/symfony-devs/-6HAQgAB2LY/uBSx8uYJQdQJ.

There are 2 parts of the PR process that this doesn't cover:

1) **Code Accuracy Review** This is something that the community can help
with, and which increases our speed and accuracy. Having a :thumbsup: or
comments from people on a ticket is a *huge* help. It's something that happens
already, but it's not part of any formal process.

2) **English Review** Also something the community can help with. Even if
you don't understand the code, if your English is good, proofreading for
English accuracy is great. I also proofread things, but I don't catch everything :).

While formalizing things, I want to keep the barrier to entry low. If someone
finds a small typo and creates a PR to the wrong branch and without this
format, I think that's still great!

If you have any feedback specifically about this PR and what we can add to
help document the process, please let me know!

Thanks!