Add some admin/emake comments

admin/emake

@@ -1,5 +1,14 @@
 #!/bin/bash
 
+# This script is meant to be used as ./admin/emake, and will compile
+# the Emacs tree with virtually all of the informational messages
+# removed, and with errors/warnings highlighted in red.  It'll give a
+# quick overview to confirm that nothing has broken, for instance
+# after doing a "git pull".  It's not meant to be used during actual
+# development, because it removes so much information that commands
+# like `next-error' won't be able to jump to the source code where
+# errors are.
+
 cores=1
 
 # Determine the number of cores.