Add comments to explain how the libtool versioning works

A lot easier than having to google this every time. And it avoids people using
the versioning in a different way (there are multiple ways on how the version
can be used).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Bump library version to 2:0:1

evemu_has_bit() added in 63985d71559117e9dee8880866f4c2bef20975a7 requires a
bump.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>