* LICENSE: add title and copyright notice
The title is not legally mandated, but it's recommended in the license template text (see http://choosealicense.com/licenses/isc/ and https://opensource.org/licenses/isc-license).
The copyright notice is required, IIRC.
* fix date in copyright notice
* add notice about additional licensing terms
+The license below applies to most, but not all content in this project.
+Files with different licensing and authorship terms are marked as such.
+That information must be considered when ensuring licensing compliance.
+
+ISC License
+
+Copyright (c) 2008-2017, Vincent Bernat <vincent@bernat.im>
+
Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.