docs: add async chat (zulip) links

see:
https://lists.nongnu.org/archive/html/mingw-cross-env-list/2019-06/msg00002.html

README.md

@@ -5,6 +5,8 @@
 [license-page]: LICENSE.md
 [license-badge]: https://img.shields.io/badge/License-MIT-brightgreen.svg
 
+[![Async Chat (Trial))](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://mxe.zulipchat.com/)
+
 MXE (M cross environment) is a GNU Makefile that compiles a cross
 compiler and cross compiles many free libraries such as SDL and
 Qt. Thus, it provides a nice cross compiling environment for

docs/index.html

@@ -21,6 +21,7 @@
         <li><a href="#download"         >Download</a></li>
         <li><a href="https://lists.nongnu.org/mailman/listinfo/mingw-cross-env-list">Mailing List</a></li>
         <li><a href="https://github.com/mxe/mxe/issues">Issue Tracker</a></li>
+        <li><a href="https://mxe.zulipchat.com/">Async Chat (Trial)</a></li>
     </ul>
     <ul>
         <li><a href="#requirements"     >Requirements</a></li>