PostGIS Installation pre-requisites

[marielle-nec]

It's not very clear that one needs to have post Postgres and PostGIS already installed on the users computer before following the steps to create or update a Rails project/database using the gem. Please consider adding this to prevent others from wasting a chunk of time figuring it out :)

[teeparham]

I agree it'd be nice to have set-up instructions for all the pre-requisites for all supported platforms.

Pull requests are welcome.

[keithdoggett]

Fixed by #334