Fix dead links & add open source license info

Author: PaulStoffregen

keywords.txt

@@ -5,5 +5,5 @@ maintainer=Arduino <[email protected]>
 sentence=Helpers for MadgwickAHRS algorithm
 paragraph=This library wraps the official implementation of MadgwickAHRS algorithm to get orientation of an object based on accelerometer and gyroscope readings
 category=Data Processing
-url=http://arduino.cc/en/Reference/XXXXXX
+url=https://github.com/arduino-libraries/MadgwickAHRS
 architectures=*

library.properties

@@ -3,7 +3,11 @@
 //=====================================================================================================
 //
 // Implementation of Madgwick's IMU and AHRS algorithms.
-// See: http://www.x-io.co.uk/node/8#open_source_ahrs_and_imu_algorithms
+// See: http://www.x-io.co.uk/open-source-imu-and-ahrs-algorithms/
+//
+// From the x-io website "Open-source resources available on this website are
+// provided under the GNU General Public Licence unless an alternative licence
+// is provided in source."
 //
 // Date			Author          Notes
 // 29/09/2011	SOH Madgwick    Initial release

src/MadgwickAHRS.cpp

@@ -3,7 +3,11 @@
 //=====================================================================================================
 //
 // Implementation of Madgwick's IMU and AHRS algorithms.
-// See: http://www.x-io.co.uk/node/8#open_source_ahrs_and_imu_algorithms
+// See: http://www.x-io.co.uk/open-source-imu-and-ahrs-algorithms/
+//
+// From the x-io website "Open-source resources available on this website are
+// provided under the GNU General Public Licence unless an alternative licence
+// is provided in source."
 //
 // Date			Author          Notes
 // 29/09/2011	SOH Madgwick    Initial release