Add description of hybrid36.

Addresses https://github.com/jensengroup/propka-3.1/pull/33#discussion_r429108127

propka/atom.py

@@ -2,7 +2,6 @@
 import string
 import propka.lib
 import propka.group
-# TODO - what is hybrid36?
 from . import hybrid36
 
 

propka/hybrid36.py

@@ -1,3 +1,7 @@
+"""Provides an alternative PDB format that can transparently encode larger atom numbers.
+
+http://cci.lbl.gov/hybrid_36/
+"""
 import string
 
 _hybrid36_upper_chars = set(string.ascii_uppercase)