Clarify license

[ryandesign]

README.md says:

marisa-trie/README.md

Lines 58 to 61 in 006020c

	#### Source code license
	* The BSD 2-clause License
	* The LGPL 2.1 or any later version

It is not clear whether this means one must obey the terms of both:

• BSD-2-clause and LGPL-2.1-or-later

or either:

• BSD-2-clause or LGPL-2.1-or-later

COPYING.md says:

marisa-trie/COPYING.md

Line 3 in 006020c

libmarisa and its command line tools are dual-licensed under the BSD 2-clause license and the LGPL.

I've often seen the term "dual-licensed" used to mean one can choose one license or the other, but it would be good to specify the interpretation you intended explicitly.