Clarify how to configure g2g and Graphite itself

[Travis Bear]

Add some additional documentation to cover this question

from grinder-use mailing list:

Hello Travis,

Good work on the new release!

One thing we have be struggling with is the configuration of both g2g and graphite to ensure the most granular data without overwhelming either g2g or graphite/whisper/etc.

Would it be possible for you to share the configuration your have (especially around retentions and data points)?

Thanks, Ben