[MRG] FAQ: what to do with strings (and graphs and trees and ...)

Quick write-up to prevent more questions like [this one](http://stackoverflow.com/q/26370056/166749), amounting to "I have strings, what do I do?" It looks like non-stats-savvy users can really not find the answer to this question in our docs.

Also includes the trick I proposed at #3737 for string metric. It doesn't really solve the issue, but it presents the workaround.