docs: add example instances and changelog

examples/php43.cnf

@@ -0,0 +1,24 @@
+c pigeonhole PHP(4,3): 4 pigeons, 3 holes, UNSAT
+p cnf 12 22
+1 2 3 0
+4 5 6 0
+7 8 9 0
+10 11 12 0
+-1 -4 0
+-1 -7 0
+-1 -10 0
+-4 -7 0
+-4 -10 0
+-7 -10 0
+-2 -5 0
+-2 -8 0
+-2 -11 0
+-5 -8 0
+-5 -11 0
+-8 -11 0
+-3 -6 0
+-3 -9 0
+-3 -12 0
+-6 -9 0
+-6 -12 0
+-9 -12 0