Add documentation on using external fingerprints in indigo/bingo

`Indigo.loadFingerprintFromDescriptors()` and `Indigo.loadFingerprintFromBuffer()` exist, as does `Bingo.insertWithExtFP()`, but the documentation on how to use these to implement a bingo DB with external fingerprints is lacking.

I have some vectors of measurements thatt I'd like to convert into fingerprints in order to perform similarity lookup using bingo, but I can't determine exactly how (even after perusing the source for a while).

```
descriptors = ?
fp = indigo.loadFingerprintFromDescriptors(descriptors, ?, ?)
bingodb.insertWithExtFP(?, fp)
```

What should be where the question marks are? What's the reasoning/process in converting an array of (normalized) floats into a vector of bits?