Added link to Slack channel

Author: mholt

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 ## Contributing to Caddy
 
+**[Join us on Slack](https://gophers.slack.com/messages/caddy/)** to chat with other Caddy developers! ([Request an invite](http://bit.ly/go-slack-signup), then join the #caddy channel.)
+
 This project gladly accepts contributions. Interested users are encouraged to get involved by [opening issues](https://github.com/mholt/caddy/issues) with their ideas, questions, and bug reports. Bug reports should contain clear instructions to reproduce the problem and state expected behavior.
 
 For small tweaks and bug fixes, feel free to submit [pull requests](https://github.com/mholt/caddy/pulls) at any time. For new features or to change existing behavior, please open an issue first to discuss it and claim it. This prevents overlapping efforts and also keeps the project in-line with its goals.

README.md

@@ -114,6 +114,8 @@ For more documentation, please view [the website](https://caddyserver.com/docs).
 
 ## Contributing
 
+**[Join us on Slack](https://gophers.slack.com/messages/caddy/)** to chat with other Caddy developers! ([Request an invite](http://bit.ly/go-slack-signup), then join the #caddy channel.)
+
 This project gladly accepts contributions. Interested users are encouraged to get involved by opening issues with their ideas, questions, and bug reports. Bug reports should contain clear instructions to reproduce the problem and state expected behavior.
 
 For small tweaks and bug fixes, feel free to submit pull requests at any time. For new features or to change existing behavior, please open an issue first to discuss it and claim it. This prevents overlapping efforts and also keeps the project in-line with its goals.