Update layer 3 protocol

[spiccinini]

We should state that the default is babel. Morover we can say that different layer 3 protocols can be used.
At least we should update howitworks.html

[ilario]

yesss

[henmohr]

Good!

My sugestions:
Why this change happened;
How to migrate your nodes to babel (is it better? my nodes will work with BMX6 and Babel? can i have both? need a fresh firmware or can i update via opkg? and so on..);