Add facet labels list back in

Adds more context as to why refinements are needed
Uses an example list similar to features above

doc/development/contributing/issue_workflow.md

@@ -207,6 +207,13 @@ Examples of feature labels are `~wiki`, `~ldap`, `~api`, `~issues`, `~"merge req
 
 Feature labels are all-lowercase.
 
+### Facet labels
+
+To track additional information or context about created issues, developers may
+add _facet labels_. Facet labels are also sometimes used for issue prioritization
+or for measurements (such as time to close). An example of a facet label is the
+~customer label, which indicates customer interest.
+
 ### Department labels
 
 The current department labels are: