This commit extends the documentation in the following ways:
- enumerating the required capabilities
- showing a short description of each flag/option
- porting the documentation from the C version on how to do xattr
  mapping
- adding links to relevant sections/documents, such as
  contributing, running as non-root and xattr mapping
- adding the uid/gid mapping requirement for user namespaces
- adding a shot FAQ section

Signed-off-by: German Maglione <gmaglione@redhat.com>