unposted: Move a new incompatibility notice.

The change in question was (deliberately) not merged to master before the 5.8
release.  Therefore, it is a "post 5.8" change, even though it was written
when 5.7.1 was the latest release.

ChangeLog

@@ -1,5 +1,7 @@
 2020-03-07  Daniel Shahaf  <danielsh@apache.org>
 
+	* unposted: README: Move a new incompatibility notice.
+
 	* unposted: Test/V01zmodload.ztst: Fix trailing whitespace in
 	test expectations.
 

README

@@ -31,13 +31,16 @@ Zsh is a shell with lots of features.  For a list of some of these, see the
 file FEATURES, and for the latest changes see NEWS.  For more
 details, see the documentation.
 
-Incompatibilities since 5.7.1
------------------------------
+Incompatibilities since 5.8
+---------------------------
 
 Build-time change: The default value of the --enable-gdbm configure
 argument has changed from "yes" to "no".  Thus, the zsh/db/gdbm module will
 not be built unless --enable-gdbm is passed explicitly.
 
+Incompatibilities since 5.7.1
+-----------------------------
+
 The history expansion !:1:t2 used to be interpreted such that the 2
 was a separate character added after the history expansion.  Now
 it is an argument to the :t modifier.