Add warning into the docs about potential for clustered sequences to be mislabeled

[wasade]

This shouldn't be an issue in practice as the emphasis is tree structure and branch length. However, the following situation is at least possible, and could lead to sequences in the ghost tree having a different taxon label than what they have in the reference. As such, it might be a good idea to include a warning about using a taxonomy derived off of the ghost-tree.

If OTU A has 10 sequences, 6 of which are labeled FOO, 4 as BAR, and OTU B has 1000 sequences, 501 are labeled FOO, 499 as BAZ, the resulting OTU will represent 1010 sequences, of which only 501 are FOO.