Update README.

README.md

@@ -5,9 +5,16 @@ This module creates metadata reports on a new tab for every bundle, for example:
 
 Adding `/download` to that URL downloads a CSV report.
 
-The report lists all fields, their types and properties, and (if the appropriate modules are available)
+The report lists all fields, their types and properties, a number representing a usage count, and (if the appropriate modules are available)
 * whether the field is indexed in Search API
 * whether the field has a Facet.
 
+Notes about usage counts:
+
+* For regular fields, the usage count represents the number of entries in the field table. This counts each "delta" as an individual usage, so fields with high repeatability will have higher usage counts than the number of distinct entities using that field. 
+* Further to that, if revisions are being stored, then the usage count will include past revisions as well. Therefore the usage count will show a non-zero number if any revision uses a field, even if that field has been emptied out in all recent revisions.
+* Further to that, if a field is shared across bundles, the usage count will include usage across all bundles, not just the one being profiled.
+* For built-in properties, such as title, name, and description, the usage count will show count each entity in the bundle, regardless of whether or not that field is populated. 
+
 ## License
 GPLv3