added nightly.mozilla.org mention to README.txt, pushing to a CLOSED TREE, a=davidb

README.txt

@@ -23,3 +23,5 @@ Keep in mind that nightly builds, which are used by Mozilla developers for
 testing, may be buggy. Firefox nightlies, for example, can be found at:
 
     ftp://ftp.mozilla.org/pub/firefox/nightly/latest-trunk/
+            - or -
+    http://nightly.mozilla.org/