Add more explanation of what drm_cscope does

drm_cscope.el provides functions to create `cscope-master-info-table` and set a buffer's `cscope-id`. These depend on Bill Carpenter's cscope.el which is a little hard to find. I found a copy at https://www.emacswiki.org/emacs/cscope.el. Document this in the README or drm_cscope.el itself.