doc: use HTTPS for links where possible

Provide encrypted links to freenode IRC.

PR-URL: https://github.com/nodejs/node/pull/6019
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

README.md

@@ -122,8 +122,8 @@ Node.js from source.
 * [CODE_OF_CONDUCT.md](./CODE_OF_CONDUCT.md)
 * [CONTRIBUTING.md](./CONTRIBUTING.md)
 * [GOVERNANCE.md](./GOVERNANCE.md)
-* IRC (general questions): [#node.js on Freenode.net](http://webchat.freenode.net?channels=node.js&uio=d4)
-* IRC (node core development): [#node-dev on Freenode.net](http://webchat.freenode.net?channels=node-dev&uio=d4)
+* IRC (general questions): [#node.js on Freenode.net](https://webchat.freenode.net?channels=node.js&uio=d4)
+* IRC (node core development): [#node-dev on Freenode.net](https://webchat.freenode.net?channels=node-dev&uio=d4)
 * [nodejs/node on Gitter](https://gitter.im/nodejs/node)
 
 ## Security