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>