ADD a STRING= STRING-EQUAL example in equality-distinctions.lsp

Issue #21

Examples took from:
  http://www.lispworks.com/documentation/HyperSpec/Body/f_stgeq_.htm/

The clarify quote is a rewrite from Google Common Lisp Style Guide:

Lisp provides four general equality predicates: EQ, EQL, EQUAL,
and EQUALP, which subtly vary in semantics. Additionally,
Lisp provides the type-specific predicates =, CHAR=, CHAR-EQUAL,
STRING=, and STRING-EQUAL.

--
Juanito Fatas