Add Documentation for using @addlogprob! correctly

[itsdfish]

Issue #201 describes a problem with using @addlogprob!. The solution was to wrap it in a conditional:

if DynamicPPL.leafcontext(__context__) !== DynamicPPL.PriorContext() 
       @addlogprob! ....
end

This would be great to add to the documentation.

[nikola-sur]

Thank you for this comment! This would be very useful to add to the documentation because other users seem to have run into this. Feel free to create a PR or I can set that up later.

[itsdfish]

Seems easy enough. I will submit a PR in a few days.