doc: add python-gflags LICENSE block

PR-URL: #464
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

LICENSE

@@ -246,6 +246,38 @@ The externally maintained libraries used by io.js are:
     # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   """
 
+- python-gflags is located at tools/gflags.  Its license follows:
+  """
+    # Copyright (c) 2006, Google Inc.
+    # All rights reserved.
+    #
+    # Redistribution and use in source and binary forms, with or without
+    # modification, are permitted provided that the following conditions are
+    # met:
+    #
+    #     * Redistributions of source code must retain the above copyright
+    # notice, this list of conditions and the following disclaimer.
+    #     * Redistributions in binary form must reproduce the above
+    # copyright notice, this list of conditions and the following disclaimer
+    # in the documentation and/or other materials provided with the
+    # distribution.
+    #     * Neither the name of Google Inc. nor the names of its
+    # contributors may be used to endorse or promote products derived from
+    # this software without specific prior written permission.
+    #
+    # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+    # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+    # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+    # A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+    # OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+    # SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+    # LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+    # DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+    # THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+    # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+    # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+  """
+
 - tools/cpplint.py is a C++ linter. Its license follows:
   """
     # Copyright (c) 2009 Google Inc. All rights reserved.