Update readline/README to mention patchlevel

This updates readline/README to mention the current patchlevel, and
the "git am" approach to importing new upstream patches.

readline/ChangeLog
2020-06-30  Tom Tromey  <tom@tromey.com>

	* README: Update instructions.

readline/ChangeLog

@@ -1,3 +1,7 @@
+2020-06-30  Tom Tromey  <tom@tromey.com>
+
+	* README: Update instructions.
+
 2019-11-15  Tom Tromey  <tromey@adacore.com>
 
 	* configure, Makefile.in: Rebuild.

readline/README

@@ -12,3 +12,7 @@ carries.
 
 If your import removes the need for a local patch, please remember to
 update this file.
+
+Individual upstream readline patches can be directly imported using
+"git am".  You can see the current patch level by looking at
+readline/patchlevel.