clarify mysql privileges required for wallabag

i am just guessing here, but it would be good to clarify this
`wallabag`, not `Wallabag.  
`vous-même`, instead of `vous-mêmes`.

Can you a part for postgresl, just `### PostgreSQL` with `TODO` below, please? 

And can you move `wallabag est maintenant installé` before `## Connexion` ?

When it'll be good, I'll merge your pull request. Thank you for it!