Mention literal-include's value for avoiding copy/pasted examples

[danielballan]

This issue is lifting an unaddressed review comment by @mrakitin in #82

I think it's worth to mention that this approach also solves the synchronization issue with the multiple sources of code.

I can't quite figure out how to phrase this succinctly. We have examples in https://github.com/caproto/caproto/blob/master/doc/source/iocs.rst where we link to the source of modules located in the package itself. That'a nice example. Gotta figure how to explain it to a newcomer.