Create CONTRIBUTORS.md

Long overdue implementation of acknowledgements of contributions to rdflib-sqlalchemy code.

Author: gjhiggins

CONTRIBUTORS.md

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+> “Imagine your beautiful program locked up in a drawer and never used again. Now imagine it being used effectively and with pleasure by many people. Which dream gives you satisfaction?” - Eric S. Raymond
+
+Contributors to the rdflib-sqlalchemy project:
+----------------------------------------------
+
+* Françoise Conil - https://github.com/fconil
+* Adam Ever-Hadani - https://github.com/adamhadani
+* Yves-Marie Haussonne - https://github.com/ymph 
+* Graham Higgins - https://github.com/gjhiggins
+* Ryan Shaw - https://github.com/rybesh