Mention helpful debugging hints in the readme

[SimonSimCity]

As people have asked in the channels of the meteor community and I myself were often unsure on how to do it: Are there commands and known things to make debugging an instance where the reactivity is not working, easier?

Stuff that immediately comes to my mind is:

• Check the command line whether the Meteor application could connect to the redis server
• Monitor the messages on your redis server using PSUBSCRIBE *

Are there any other things a person could do to get more insights about what part of the configuration/network/etc is wrong?

I'd like to make up a section for the wiki which we can refer to if someone asks in the future.