Updates to Readme for JMX Remote Plugin

It was brought to my attention that the directions weren't clear on setting the GUID so that the dashboards would be editable, which is a necessity for the JMX Remote capability.
Remove range slice data from read latency metric calculation

This was causing read latency graphs to not match up with Opscenter.
Range slice shoould have separate metric data instead.

Typical read metric data from an idle cluster pre-patch:
[3.9878362E-4,2,0.0,3.9878362E-4,1.5902837E-7]

Typical read metric data from an idle cluster post-patch:
[6.023493E-4,1,6.023493E-4,6.023493E-4,3.6282466E-7]

The count of 2 was causing the average and minimum values to be off.

https://docs.newrelic.com/docs/plugin-developer-resources/metric-data-for-the-plugin-api#timeslice
"Count of the number of events this value represents over the time
period; the average is calculated by dividing total by count"
Switch from OneMinuteRate to Mean for latency metrics

OneMinuteRate counts the number of requests per second over the last one
minute, while Mean measures the average latency over the last minute, which
comes much closer to the values graphed by Opscenter.

https://groups.google.com/forum/#!msg/nosql-databases/a6P12QVbd8o/wulNbttk-A0J
https://github.com/dropwizard/metrics/blob/master/metrics-core/src/main/java/com/codahale/metrics/Metered.java#L45
https://github.com/dropwizard/metrics/blob/master/metrics-core/src/main/java/com/codahale/metrics/Snapshot.java#L170

It's generally preferable to use the median (50thPercentile) instead of
the mean, but those values don't match well with Opscenter, based on my
testing. Being closed-source, it's hard to tell how exactly Opscenter
determines its metrics.
Merge pull request #29 from jbarbuto/remove_range_slice_from_read_latency

Remove range slice data from read latency metric calculation
Merge pull request #31 from jbarbuto/use_mean_for_latency_metrics

Switch from OneMinuteRate to Mean for latency metrics
Merge pull request #23 from sschwartzman/patch-1

Updates to Readme for JMX Remote Plugin