docs/vcs_support: add information on importing repositories

This change directly relates to issue #254.

Some rework of the original patch done by Thomas De Schampheleire:
- generalize to cover both Mercurial and Git
- simplify instructions for importing via filesystem: don't impose a certain
flow as it depends on different factors: are the repos already present on
the machine, are they organized in a flat hierarchy or already in a more
complex one, ...
- invalidating cache while rescanning should not be needed when adding new
repositories