Add MIT license file; update xv6-rev2.tar.gz with LICENSE.
Other files unchanged.

LICENSE

+The xv6 software is:
+
+Copyright (c) 2006-2009 Frans Kaashoek, Robert Morris, Russ Cox,
+                        Massachusetts Institute of Technology
+
+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.
+

web/index.html

@@ -95,6 +95,8 @@ We distribute the sources in electronic form but also as
 a printed booklet with line numbers that keep everyone
 together during lectures.  The booklet is available as
 <a href="xv6-rev2.pdf">xv6-rev2.pdf</a>.
+The xv6 source code is licensed under the traditional <a href="http://www.opensource.org/licenses/mit-license.php">MIT license</a>;
+see the LICENSE file in the source distribution.
 <br><br>
 xv6 compiles using the GNU C compiler,
 targeted at the x86 using ELF binaries.