add contributors list

Author: rtfpessoa

CREDITS.md

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+# Credits
+
+This is the list of all the kind people that have contributed to the diff2html project.
+This list or ordered by first contribution.
+
+Thanks,
+@rtfpessoa
+
+----------
+
+Rodrigo Fernandes, [@rtfpessoa](https://github.com/rtfpessoa)
+
+JK Kim, [@codingtwinky](https://github.com/codingtwinky)
+
+Paulo Bu, [@pbu88](https://github.com/pbu88)
+
+Nuno Teixeira, [@nmatpt](https://github.com/nmatpt)
+
+Mikko Rantanen, [@Rantanen](https://github.com/Rantanen)