Add a rpm spec file
Add a rpm spec file

This makes it easier to build/package cgreen from git for RPM-based
distributions. This spec file is fine for Fedora 27, I haven't tested
building it on other platforms.

thoni56/cgreen,thoni56/cgreen,cgreen-devs/cgreen,thoni56/cgreen,cgreen-devs/cgreen,thoni56/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,thoni56/cgreen,cgreen-devs/cgreen