Add caveats

Author: yuvipanda

docs/topic/billing/mau.md

@@ -8,6 +8,13 @@ We define an active user as someone who has:
 2. Is not our health check service account `deployment-service-check`
 3. Is not a 2i2c employee (for GitHub & Google IDs)
 
+### Caveats
+
+1. We don't filter out 2i2c staff from hubs that use other authentication systems
+   (like KeyCloak, Auth0, Canvas, etc)
+2. We count each 'launch' on a binderhub as an active user, and we don't split
+   that out separately here.
+
 ## Generating MAU metrics
 
 You can generate MAU metrics for all clusters by running `deployer generate mau YYYY-MM`. You can generate a csv file with the output by passing a filename to the `--csv-file` parameter.