Clarify how usage ping opt in rate is calculated

Merged Jerome Z Ng requested to merge jeromezng-master-patch-23275 into master

Why is this change being made?

This MR updates the Is it any good handbook page with details about how the usage ping opt in rate is calcualted.

This is a follow up to @sytses's slack question.

The simple explanation is that we take the total number of users reporting usage ping (Versions app) and divide it by the total number of licensed users (License app).

Attaching a screenshot of the SQL from the Sisense dashboard: https://app.periscopedata.com/app/gitlab/545874/Customers-and-Users-with-Usage-Ping-Enabled

Screen_Shot_2020-08-14_at_9.41.58_PM

Author Checklist

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