added new hook info to docs

doc/plugin.txt

@@ -120,6 +120,7 @@ List of hooks
   read_body_top                   src/read_body.php
   read_body_bottom                src/read_body.php
   html_bottom                     src/read_body.php
+  read_body_header                src/read_body.php
   search_before_form              src/search.php
   search_after_form               src/search.php
   search_bottom                   src/search.php