Distribute src/ext/README.

src/ext/include.am

@@ -1,6 +1,8 @@
 
 AM_CPPFLAGS += -I$(srcdir)/src/ext -Isrc/ext
 
+EXTRA_DIST += src/ext/README
+
 EXTHEADERS = \
   src/ext/ht.h		\
   src/ext/eventdns.h	\