docs: describe the difference between built in counter cache vs this gem

[hayksaakian]

From what I read in the readme, seems like this definitely works on embedded documents whereas its ambiguous in the docs of mongoid built in counter cache .

Thats just one example.

[jah2488]

This gem pre-dates the build in counter cache and at this point I am not familiar with the built in counter cache functionality.

[bkoski]

The built-in counter cache isn't mentioned in the mongoid.org docs but is implemented as a CounterCache module.

Seems like this gem has two key advantages:

• embedded document support
• conditional counts, i.e. if item is at status 'published'