Add a link to Jef Poskanzer's original phoon (https://acme.com/software/phoon/)

The README gives the email address and website of Jef Poskanzer's original phoon, as well as some background info about it (Written in PASCAL in 1979, ported to C). But not the link to the actual project page (https://acme.com/software/phoon/). This makes locating the original C source code for study or analysis a difficult, involved process.

In astro.py there is a link to John Walkers original version of `moontool` which the astronomical calculations are based off of. It feels natural to also provide a link to Jef Posanker's code somewhere (possibly in the README with the other info) too.