Add NOTICE.txt to comply with ASLv2 instructions

According to the Apache website for ASLv2:

  http://www.apache.org/dev/apply-license.html#new

Which states "In addition, a correct NOTICE file MUST be included in the same
directory as the LICENSE file.". This patch adds such a file.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #581 from kbarber/maint/notice

Add NOTICE.txt to comply with ASLv2 instructions