Update Readme

After seeing the exchange at
https://gis.stackexchange.com/questions/32618/how-to-build-a-geographic-database-of-gps-logs
and realising we were not being very explanatory about the users
compulsory argument.
Merge remote-tracking branch 'upstream/master'
Tidy up
Merge pull request #21 from belasco/master

A bit more documentation in the readme