Add link to pypa/python-manylinux-demo in README

[rmcgibbo]

I got the python-manylinux-demo repo fixed and running with the current docker layout. It might be a good idea to link to it in the README as a demonstration.

[ibell]

A 100% endorsement of linking to this repo. Docker environments are not obvious to the uninitiated, and this repo covers all the salient points.

[rmcgibbo]

@ibell see #56. if you can think of any text I should add beyond that, i'd be happy to add more too.

[AbdealiLoKo]

I believe this can be closed ? I do see the link in the README now