Add documentation for propagator config parameter

[jamesylgan]

#425 alludes to the difficulty users have in figuring out how to configure an application to use a specific propagator (e.g. b3).

This feature is not documented like most other config parameters in the main README of the project.

I think it would be valuable to explain, in the main project README, how the ota.propagators parameter works.

I have found the diagrams here to be helpful in understanding header propagation models for the SignalFx Java Instrumentation agent - something like that could also be referenced if these docs changes are made.