Adding messsage encoding to onboarding FAQ
Adding messsage encoding to onboarding FAQ

It took some questions and answers for me to figure out how the messages
are encoded (https://github.com/raiden-network/spec/issues/133 ).

This commit adds what I learned in the FAQ, namely, how to figure out
the format of a message.

[no ci]

hackaugusto/raiden,hackaugusto/raiden