subst-async changed to reflect Zsh 5.8 bug fix

Author: agkozak

README.md

@@ -58,6 +58,8 @@ This prompt has been tested on numerous Linux and BSD distributions, as well as
 <details>
   <summary>Here are the latest features and updates.</summary>
 
+- v3.11.3
+    + [A bug was fixed in Zsh 5.8 itself](https://www.zsh.org/mla/workers/2019/msg00252.html) that had required the `subst-async` method to do extra forking as a workaround. The code for `subst-async` has been updated to reflect the bug fix. The majority of users default to the `usr1` async method, however, which is unaffected.
 - v3.11.2
     + `zsh-async` v1.8.6 is included.
     + The behavior of `$AGKOZAK_SHOW_STASH` has been fixed.