Add CONTRIBUTORS file (#233)

CONTRIBUTORS

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+Bleach was originally written and maintained by Erik Rose and various
+contributors within and without the Mozilla Corporation and Foundation.
+
+It is currently maintained by XYZ.
+
+Maintainer emeritus:
+
+* Erik Rose
+* Will Kahn-Greene
+* Lonnen
+
+Contributors:
+
+* Cristiano Santos
+* David Huggins-Daines
+* David Li
+* Fabien LOISON
+* Igor Loskutov
+* Jam Risser
+* Jared Dillard
+* Joseph H Kennedy
+* krassowski
+* mariusschenzle
+* Mike Cooper
+* Nicholas Bollweg
+* Nick Alexander
+* Patrick Browne
+* Paul Grau
+* Robert Helmer
+* Sebastian Weigand
+* Staś Małolepszy
+* Stefan 'hr' Berder
+* s-weigand
+* Tavian Barnes
+* Thomas Khyn
+* Wim Yedema