Added docs for OpenShift setup and linked from README

Added setup docs for setting up stringer on OpenShift, and added a link in README to the OpenShift setup docs.

Information from [OpenShift Blog](https://www.openshift.com/blogs/stringer-cause-google-reader-abandoned-your-feeds), with reference to issue #266.

Credits to Marek Jelen, Nam Duong and OpenShift.