docs: add KnownVuln anti-feature

Merged username-removed-25246 requested to merge krt/fdroid-website:antifeatures into master

This AF has been added in https://gitlab.com/fdroid/fdroidserver/commit/98297278bde9df5945c6fce59567b00a792b5f86. Newlines have been added to match current scheme of the file (2 following a section heading, 3 in front).