add license and update readme

License.txt

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+The MIT License (MIT)
+
+Copyright (c) 2013 Seeed Technology 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.md

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+Grove - Digital Light Sensor
+---------------------------------------------------------
+[![Grove - Digital Light Sensor](http://www.seeedstudio.com/depot/images/product/Digital%20Light%20Sensor.jpg)](http://www.seeedstudio.com/depot/grove-digital-light-sensor-p-1281.html?cPath=25_27)
+
+This module is based on the I2C light-to-digital converter TSL2561 to transform light intensity to a digital signal. Different from traditional analog light sensor, as [Grove - Light Sensor](http://www.seeedstudio.com/depot/grove-light-sensorp-p-1253.html?cPath=144_148), this digital module features a selectable light spectrum range due to its dual light sensitive diodes: infrared and full spectrum. 
+ 
+You can switch between three detection modes to take your readings. They are infrared mode, full spectrum and human visible mode. When running under the human visible mode, this sensor will give you readings just close to your eye feelings.
+
+
+# Usage:
+
+This is an Arduino Library of Grove - Digital Light Sensor.
+
+### Initialization:
+
+    TSL2561.init(); 
+
+### Get Data
+
+    unsigned long calculateLux(unsigned int iGain, unsigned int tInt,int iType);
+
+
+<br>
+For more information, please refer to [wiki page](http://www.seeedstudio.com/wiki/Grove_-_Digital_Light_Sensor).
+
+    
+----
+
+
+This software is written by loovee [luweicong@seeedstudio.com](luweicong@seeedstudio.com "luweicong@seeedstudio.com") for seeed studio<br>
+and is licensed under [The MIT License](http://opensource.org/licenses/mit-license.php). Check License.txt for more information.<br>
+
+Contributing to this software is warmly welcomed. You can do this basically by<br>
+[forking](https://help.github.com/articles/fork-a-repo), committing modifications and then [pulling requests](https://help.github.com/articles/using-pull-requests) (follow the links above<br>
+for operating guide). Adding change log and your contact into file header is encouraged.<br>
+Thanks for your contribution.
+
+Seeed Studio is an open hardware facilitation company based in Shenzhen, China. <br>
+Benefiting from local manufacture power and convenient global logistic system, <br>
+we integrate resources to serve new era of innovation. Seeed also works with <br>
+global distributors and partners to push open hardware movement.<br>
+
+
+
+
+
+
+