add a thank you file for contributors

THANKS

@@ -0,0 +1,6 @@
+These people helped to improve OpenADK by submitting code, manuals, bug
+reports, patches or good suggestions:
+
+Thorsten Glaser <tg@mirbsd.org>
+Joerg Seitter <oss@seitter.net>
+Phil Sutter <phil@nwl.cc>