Bundle the licence for the included colour scheme

As the code for gene set tracks in the Oncoprint includes a colour
scheme as per a license that requires maintaining the copyright notice,
include this notice according to the conventions previously used in the
cBioPortal master/backend project.

OPEN-SOURCE-DOCUMENTATION

@@ -0,0 +1,51 @@
+Open Source bundled in this project
+
+This document contains the licenses and notices for open source software bundled in this product.
+With respect to the free/open software listed in this document, if you have any questions or
+wish to receive a copy of the source code to which you are entitled under the applicable
+free/open source license(s) (such as GNU Lesser/General Public License),
+please visit our https://github.com/cBioPortal/cbioportal-frontend or contact us at
+[email protected].
+
+* ColorBrewer scheme: PiYG
+
+ColorBrewer scheme: PiYG
+------------------------
+Available under license:
+
+    Apache-Style Software License for ColorBrewer software and
+    ColorBrewer Color Schemes
+
+    Copyright (c) 2002 Cynthia Brewer, Mark Harrower, and The
+    Pennsylvania State University.
+
+    Licensed under the Apache License, Version 2.0 (the "License"); you
+    may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+    implied. See the License for the specific language governing
+    permissions and limitations under the License.
+
+    Redistribution and use in source and binary forms, with or without
+    modification, are permitted provided that the following conditions
+    are met:
+    1. Redistributions as source code must retain the above copyright
+    notice, this list of conditions and the following disclaimer.
+    2. The end-user documentation included with the redistribution, if
+    any, must include the following acknowledgment:
+    "This product includes color specifications and designs developed by
+    Cynthia Brewer (http://colorbrewer.org/)."
+    Alternately, this acknowledgment may appear in the software itself,
+    if and wherever such third-party acknowledgments normally appear.
+    4. The name "ColorBrewer" must not be used to endorse or promote
+    products derived from this software without prior written
+    permission. For written permission, please contact Cynthia Brewer at
+    [email protected].
+    5. Products derived from this software may not be called
+    "ColorBrewer", nor may "ColorBrewer" appear in their name, without
+    prior written permission of Cynthia Brewer.