GUACAMOLE-136: Add licenses for libraries bundled with guacamole-auth-duo.

extensions/guacamole-auth-duo/src/licenses/DISCLAIMER

@@ -0,0 +1,7 @@
+Apache Guacamole is an effort undergoing incubation at The Apache Software
+Foundation (ASF). Incubation is required of all newly accepted projects until a
+further review indicates that the infrastructure, communications, and decision
+making process have stabilized in a manner consistent with other successful ASF
+projects. While incubation status is not necessarily a reflection of the
+completeness or stability of the code, it does indicate that the project has
+yet to be fully endorsed by the ASF.