Change php upload limit + explain it in the readme

[lapineige]

See : https://github.com/YunoHost-Apps/pixelfed_ynh/issues/118#issuecomment-700513503

PHP default upload limit is too low¹ for Pixelfed default value. Also users could change Pixelfed value without any visible effect, because of PHP limit.

¹ it seems to be 2MB by default, which is limited for images and well too low for videos…

What needs to be done:

• [ ] Add new php upload limit (with a file located in Pixelfed folder ?)
• [x] Add new nginx upload limit
• [ ] Explain the readme how to change that value (note: that one is an easier task, any contribution, even if not complete, would be appreciated ;-) :-) )