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