fix: update Slack references to Discord (#427)

Signed-off-by: anablaz <[email protected]>

README.md

@@ -80,8 +80,8 @@ netlify dev -d build
   <a href="./LICENSE">
     <img src="https://img.shields.io/github/license/accordproject/cicero?color=bright-green" alt="GitHub license">
   </a>
-  <a href="https://accord-project-slack-signup.herokuapp.com/">
-    <img src="https://img.shields.io/badge/Accord%20Project-Join%20Slack-blue" alt="Join the Accord Project Slack"/>
+  <a href="https://discord.gg/Zm99SKhhtA">
+    <img src="https://img.shields.io/badge/Accord%20Project-Join%20Discord-blue" alt="Join the Accord Project Discord"/>
   </a>
 </p>
 
@@ -99,7 +99,7 @@ The Accord Project technology is being developed as open source. All the softwar
 
 Find out what’s coming on our [blog][apblog].
 
-Join the Accord Project Technology Working Group [Slack channel][apslack] to get involved!
+Join the Accord Project Technology Working Group [Discord channel][apdiscord] to get involved!
 
 For code contributions, read our [CONTRIBUTING guide][contributing] and information for [DEVELOPERS][developers].
 
@@ -123,7 +123,7 @@ Copyright 2018-2019 Clause, Inc. All trademarks are the property of their respec
 [apmain]: https://accordproject.org/ 
 [apblog]: https://medium.com/@accordhq
 [apdoc]: https://docs.accordproject.org/
-[apslack]: https://accord-project-slack-signup.herokuapp.com
+[apdiscord]: https://discord.gg/Zm99SKhhtA
 
 [contributing]: https://github.com/accordproject/cicero-template-library/blob/master/CONTRIBUTING.md
 [developers]: https://github.com/accordproject/cicero-template-library/blob/master/DEVELOPERS.md