Add license file

LICENSE.md

@@ -0,0 +1,41 @@
+The PX4 firmware is licensed generally under a permissive 3-clause BSD license. Contributions are required
+to be made under the same license. Any exception to this general rule is listed below.
+
+	/****************************************************************************
+	 *
+	 *   Copyright (c) 2012-2014 PX4 Development Team. All rights reserved.
+	 *
+	 * Redistribution and use in source and binary forms, with or without
+	 * modification, are permitted provided that the following conditions
+	 * are met:
+	 *
+	 * 1. Redistributions of source code must retain the above copyright
+	 *    notice, this list of conditions and the following disclaimer.
+	 * 2. 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.
+	 * 3. Neither the name PX4 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.
+	 *
+	 ****************************************************************************/
+
+
+  - PX4 middleware: BSD 3-clause
+  - PX4 flight control stack: BSD 3-clause
+  - NuttX operating system: BSD 3-clause
+  - Exceptions: Currently only this [400 LOC file](https://github.com/PX4/Firmware/blob/master/src/lib/external_lgpl/tecs/tecs.cpp) remains LGPL, but will be replaced with a BSD implementation.