Update readme.md

Author: GrantEdwards

readme.md

@@ -2,6 +2,9 @@
 
 This extension allows the user to configure custom CSS that will be
 added to all email messages (including replies and forwards) when they
-are first "composed".
-
+are "composed".
 
+Each time an email is opened in a compose window (write, reply, forward),
+the custom CSS will be added to a newly created <style> element in the
+email's head. If the head already contains a <style> element that was
+added by this extension, it is replaced.