Add CREDITS file

CREDITS.md

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+Credits
+=======
+
+Rudy Matela:
+  original implementation of FitSpec.
+
+Colin Runciman:
+  improvements in library interface and code;
+  set and digraph examples;
+  the name "FitSpec".
+
+Thanks to:
+
+* Jonas Duregård
+  for presenting a black-box mutation testing technique at HIW2014
+   inspiring me (Rudy) to start working on it
+   (cf. https://www.youtube.com/watch?v=ROKxri62WYQ);
+  for discussions;
+  for an alternative way to enumerate mutants (see patterns folder).
+
+* Nick Smallbone
+  for the "Heap" example (adapted from the QuickSpec tool package).