Document how to include bootstrap support in a tree

[huddlej]

A common task in phylogenetics is to inspect the bootstrap support for individual nodes in a tree. Now that Augur supports passing (some) bootstrap confidence values through to the Auspice JSON, we should document how to generate these values with augur tree and modifications to the Auspice config. A simple howto guide with the commands from the PR linked above and some screenshots would be a minimal solution.