SPDX: document dual license notation

In [1] we discussed how we should deal with dual (or, more generally,
multiple) licensed files.  Add this to  Licenses/README  so it's
properly documented.

[1] http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/166518

Signed-off-by: Wolfgang Denk <wd@denx.de>
[trini: Add the word 'list' to the end of the line, per Stephen Warren's
feedback]
Signed-off-by: Tom Rini <trini@ti.com>

Licenses/README

@@ -37,6 +37,18 @@ replaced by a single line:
 
 	SPDX-License-Identifier:	GPL-2.0+
 
+Ideally, the license terms of all files in the source tree should be
+defined by such License Identifiers; in no case a file can contain
+more than one such License Identifier list.
+
+If a "SPDX-License-Identifier:" line references more than one Unique
+License Identifier, then this means that the respective file can be
+used under the terms of either of these licenses, i. e. with
+
+	SPDX-License-Identifier:	GPL-2.0+	BSD-3-Clause
+
+you can chose between GPL-2.0+ and BSD-3-Clause licensing.
+
 We use the SPDX Unique License Identifiers here; these are available
 at [2].