Add a FAQ section in the documentation

[mwouts]

It would be nice to have a FAQ in the documentation, plus a link to that in the README. The FAQ should be easy to follow, even by a person that had never heard of Jupytext.

Among the questions there I'd like to see

• What changes in Jupyter when I install Jupytext? A1: you gain the possibility to pair notebooks, but unless you pair a notebook, they continue to work in exactly the same way. A2: Scripts and Markdown files get a notebook icon. In Jupyter lab they gain an 'open with notebook' item in the contextual menu in JupyterLab. In Jupyter Notebook they open as notebooks by default
• Can I use Jupytext with JupyterHub? A: Yes. Install it globally or locally (pip install jupytext --user). If it's installed globally and that a specific user doesn't want it, he can deactivate the server extension (and the other extensions).
• Can I user Jupytext with cloud notebook editors? A: Not in their interface (yet?). But you can use the CLI Document how to use Jupytext with Hub/Colab/Saturn/Azure/nteract etc #256
• Can I use Jupytext on Binder? A: Yes: jupytext in requirements + install lab extension
• How do pair notebooks work? A: inputs + outputs in ipynb, input and metadata only in the text files, inputs in the text file have a precedence over those in the ipynb file
• Some metadata is missing in the paired file! A: tell about filters. A:filtered metadata is preserved in the ipynb file
• I get a message 'the notebook has changed on disk'? A: Be faster than autosave... or turn it off. A: Use an editor that can show the diff memory vs disk.
• I love Jupyter widgets, does Jupytext support them? A: Yes. In the paired files, Jupytext saves every part of the notebook that was writen by the user (cell inputs and metadata).
• Is Jupytext safe to use? A: We try our best! Round trip is well tested: covered with 1000+ tests. A: Safer than without as you now see the diffs before commit A: Editing the same document in two editors simultaneously must be done with attention, please be careful. Still it should be hard to make a mistake (we check timestamps, etc)

[mwouts]

The FAQ could also include a short mention of #267 #268 #263 #251