Add license text for the whole package

MIT-LICENSE

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+Copyright (c) 2013 Luminoso Technologies, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.

README.txt

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 Tools for working with word frequencies from various corpora.
 
 Author: Rob Speer
+
+## License
+
+`wordfreq` is freely redistributable under the MIT license. See
+`MIT-LICENSE.txt`.
+
+It contains data extracted from Google Books Ngrams
+(http://books.google.com/ngrams). The terms of use of this data are:
+
+    Ngram Viewer graphs and data may be freely used for any purpose, although
+    acknowledgement of Google Books Ngram Viewer as the source, and inclusion
+    of a link to http://books.google.com/ngrams, would be appreciated.
+
+It also contains data from the University of Leeds Centre for Translation
+Studies (see http://corpus.leeds.ac.uk/list.html), which is freely
+redistributable under a Creative Commons Attribution license.