Makefile: Updating release files on orgmode.org server

Add a section on some directives that the maintainer should
only use from the orgmode.org server.

README_maintainer

@@ -131,6 +131,23 @@ from maint with this command:
 
 : make fixrelease TAG=7.13.01
 
+** Updating release files on orgmode.org server
+
+As of 2011-01-15, these directives of the Makefile are meant to be
+used /from orgmode.org server/ and will copy the release files to the
+webserver directory.
+
+- ~$ make makerelease :: creates a =RELEASE/= directory containing
+     manuals and release files (=org.tar.gz=, =org.zip=, etc.)
+
+- ~$ make sync_release :: copy the content of =RELEASE/= to the right
+     location on the server
+
+- ~$ make sync_manuals :: copy the manuals from =doc/= to the right
+     location on the server
+
+- ~$ make relup :: call the three directives described above.
+
 ** Between releases
 
 While working on master between releases, I used to use something like