add internal docs to set end user email (#16573)

for #15057

docs/Contributing/set-up-custom-end-user-email.md

@@ -0,0 +1,43 @@
+# Provide end user email address w/o relying on end user
+
+In issue #15057, we introduced a feature primarily intended for internal use
+that allows the specification of the email for the "Used by" field on the host
+details page. This feature supports macOS and Windows, but the method to define
+the email differs for each operating system.
+
+### macOS via configuration profiles
+
+When `fleetd` starts in macOS, it'll check for the user email in the enrollment profile.
+
+The enrollment profile contains multiple `<dict>` elements. Locate the one with a
+`PayloadIdentifier` equal to `com.fleetdm.fleet.mdm.apple.mdm`, and add the
+following, replacing foo@example.com with the desired user email:
+
+```xml
+<key>EndUserEmail</key>
+<string>foo@example.com</string>
+```
+
+For instance, the specific part of the payload might look like this:
+
+```xml
+<key>PayloadIdentifier</key>
+<string>com.fleetdm.fleet.mdm.apple.mdm</string>
+<key>EndUserEmail</key>
+<string>foo@example.com</string>
+```
+
+### Windows via custom installers
+
+For Windows, we implemented a hidden flag in `fleetctl` that allows you to
+define a custom user email when a package is built.
+
+If provided, `fleetd` will report the user email to the fleet server when it starts.
+
+```
+$ fleetctl package --type=msi --end-user-email=foo@example.com --fleet-url=https://test.example.com --enroll-secret=abc
+```
+
+Note: You need to build a different package per user email.
+
+<meta name="pageOrderInSection" value="1202">