Update README.md

Hey mermaid team! I noticed that the join slack button was not working. I copied the join link from https://github.com/mermaid-js/mermaid/blob/develop/README.md
mgenereu · Jun 9, 2020 · a59b2ce · a59b2ce
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commit a59b2ce
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/README.md b/README.md
@@ -1,4 +1,4 @@
-![Join our Slack!](https://img.shields.io/static/v1?message=join%20chat&color=9cf&logo=slack&label=slack)
+[![Join our Slack!](https://img.shields.io/static/v1?message=join%20chat&color=9cf&logo=slack&label=slack)](https://join.slack.com/t/mermaid-talk/shared_invite/enQtNzc4NDIyNzk4OTAyLWVhYjQxOTI2OTg4YmE1ZmJkY2Y4MTU3ODliYmIwOTY3NDJlYjA0YjIyZTdkMDMyZTUwOGI0NjEzYmEwODcwOTE)
 
 # Contributors are welome