Add example of user pref

diff --git a/README.md b/README.md
index acf671da2321511b460231b1622079767e17eb83..c3341ffe133a3752e554fc6e14fd97fef9a9b48e 100644 (file)
--- a/README.md
+++ b/README.md
@@ -4240,7 +4240,7 @@ Default preferences can be modified by the user.
 
 If a value is locked, it is also set as the default.
 
-User preferences persist across invocations of Firefox. It is the equivalent of a user setting the preference. They are most useful when a preference is needed very early in startup so it can't be set as default by policy.
+User preferences persist across invocations of Firefox. It is the equivalent of a user setting the preference. They are most useful when a preference is needed very early in startup so it can't be set as default by policy. An example of this is ```toolkit.legacyUserProfileCustomizations.stylesheets```.
 
 User preferences persist even if the policy is removed, so if you need to remove them, you should use the clear policy.