Add note about ASF HipChat room to README doc

The current NiFi README document has a Getting Help section which also mentions the ASF #NiFi HipChat room as a usually-staffed resource. As I believe that is now the preferred real-time chat option, it should be added to the MiNiFi README doc and prioritized over IRC.

From NiFi README.md:

If you have questions, you can reach out to our mailing list: dev@nifi.apache.org (archive). For more interactive conversations and chat, we're also often available in IRC: #nifi on irc.freenode.net and in #NiFi on ASF HipChat.