Add docs on map sharing to reference

A while ago in #66 / #309 @drzaeus77 added support for sharing maps between programs.

Currently it's not mentioned in the reference:

* https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md

This makes it a bit harder to figure out how to make this work.

Open docs/reference_guide.md and review the map-sharing support referenced in issues #66 and #309. Add a reference section explaining how to use map sharing between programs, so readers can discover and follow the documented workflow.

Written by the indexing model from the issue text.