Add slack links to CONTRIBUTING.md (#7713)

.github/ISSUE_TEMPLATE.md

@@ -5,7 +5,7 @@ Search open/closed issues before submitting since someone might have asked the s
 
 If you have a support request or question please submit them to one of this resources:
 
-* Slack Community: https://slack.babeljs.io/
+* Slack Community: https://babeljs.slack.com (you can sign-up at https://slack.babeljs.io/ for an invite)
 * StackOverflow: http://stackoverflow.com/questions/tagged/babeljs using the tag `babeljs`
 * Also have a look at the readme for more information on how to get support:
  https:/babel/babel/blob/master/README.md

CONTRIBUTING.md

@@ -29,12 +29,13 @@ contributing, please read the
  - Check out [the Babel Plugin Handbook](https:/thejameskyle/babel-handbook/blob/master/translations/en/plugin-handbook.md#babel-plugin-handbook) - core plugins are written the same way as any other plugin!
  - Check out [AST Explorer](http://astexplorer.net/#/scUfOmVOG5) to learn more about ASTs or make your own plugin in the browser
 - When you feel ready to jump into the Babel source code, a good place to start is to look for issues tagged with [help wanted](https:/babel/babel/labels/help%20wanted) and/or [good first issue](https:/babel/babel/labels/good%20first%20issue).
-- Follow along with what we are working on by joining our Slack, following our announcements on [Twitter](https://twitter.com/babeljs), and reading (or participating!) in our [meeting notes](https:/babel/notes).
+- Follow along with what we are working on by joining our [Slack](https://babeljs.slack.com) (you can sign-up [here](https://slack.babeljs.io/)
+ for an invite), following our announcements on [Twitter](https://twitter.com/babeljs), and reading (or participating!) in our [meeting notes](https:/babel/notes).
 - Check out our [website](http://babeljs.io/) and the [repo](https:/babel/website)
 
 ## Chat
 
-Feel free to check out the `#discussion`/`#development` channels on our [Slack](https://slack.babeljs.io). Some of us are always online to chat!
+Feel free to check out the `#discussion`/`#development` channels on our [Slack](https://slack.babeljs.io) (you can sign-up [here](https://slack.babeljs.io/) for an invite). Some of us are always online to chat!
 
 ## Developing
 

README.md

@@ -108,7 +108,7 @@ Mostly a handful of volunteers! Please check out our [team page](https://babeljs
 
 ### Looking for support?
 
-For questions and support please visit join our [Slack Community](https://slack.babeljs.io/), ask a question on [Stack Overflow](https://stackoverflow.com/questions/tagged/babeljs), or ping us on [Twitter](https://twitter.com/babeljs).
+For questions and support please visit join our [Slack Community](https://slack.babeljs.io/) (you can sign-up [here](https://slack.babeljs.io/) for an invite), ask a question on [Stack Overflow](https://stackoverflow.com/questions/tagged/babeljs), or ping us on [Twitter](https://twitter.com/babeljs).
 
 ### Is there a Babel song?