Add docs on map sharing to reference

[bobrik]

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.

[yonghong-song]

@bobrik bcc does provide a way to share maps between different BPF modules(files) in the same process. See test_clang.py test test_exported_maps (https://github.com/iovisor/bcc/blob/master/tests/python/test_clang.py#L428).

bcc also supports to share maps between different processes (though map pinning): https://github.com/iovisor/bcc/blob/master/examples/cpp/UseExternalMap.cc
the example creates a map locally in the process and incorporates it into the BPF module.
The map itself could be a pinned map shared by different processes.

Want to study these examples and help improve our documentation?