Add requirements install step to readme

[mattfullerton]

Resolves #65

[davidmsibley]

I don't think this documentation is correct. First, I think you'll want to install the requirements after the library, and then you'll need to point to the pip-requirements.txt within the ckanext-geoview library, which typically isn't the working directory when working in a production site.

[davidmsibley]

Actually, I suspect it will end up more like the ckanext-spatial install docs, by installing from source to get the pip-requirements.txt file locally.

Disclaimer: I've never worked with python/pip before, I'm just trying to install this extension and I ran into this issue.

[mattfullerton]

You're right, it actually makes little sense what I wrote. I will write back in #65. The other approach (ckanext-spatial) would of course work, but this approach (if it would work, i.e. install dependencies) would obviously be nicer for users.