Document comparison to other libraries

[jakewilliami]

One of the unfortunate things about any "new" language is a lack of consensus in good libraries to handle functionality. It looks like there are a couple of similar libraries:

• github.com/jakewarren/defang
• github.com/atakanaydinbas/gofangdefang

It would be good to document how they differ.

CC @jakewarren, @atakanaydinbas — perhaps there could be an effort to standardise.