Documentation for force-https.txt

[math-GH]

How to use the force-https.txt?

We need to add information to the documentation

With #1087 it was introduced.

But force-https.default.txt is not anymore located in /data, now it is in root folder.

[Alkarex]

The ./data/ folder is for user, writeable, custom content.

The defaults values are defined in ./force-https.default.txt and additional domains can be added in ./data/force-https.txt

See https://github.com/FreshRSS/FreshRSS/pull/1532