Improve license information

COPYING

 HarfBuzz is licensed under the so-called "Old MIT" license.  Details follow.
+For parts of HarfBuzz that are licensed under different licenses see individual
+files names COPYING in subdirectories where applicable.
 
 Copyright © 2010,2011,2012  Google, Inc.
 Copyright © 2012  Mozilla Foundation

src/hb-icu-le/COPYING

+The contents of this directory are licensed under the ICU license.
+See file license.html.

src/hb-icu-le/Makefile.am

@@ -20,6 +20,6 @@ libhb_icu_le_la_CPPFLAGS = \
 libhb_icu_le_la_LIBADD = \
 	$(ICU_LE_LIBS)
 
-EXTRA_DIST = README license.html
+EXTRA_DIST = README license.html COPYING
 
 -include $(top_srcdir)/git.mk

src/hb-ucdn/Makefile.am

@@ -13,6 +13,6 @@ libhb_ucdn_CPPFLAGS = \
 	-I$(top_builddir)/src
 libhb_ucdn_la_LIBADD =
 
-EXTRA_DIST = README
+EXTRA_DIST = README COPYING
 
 -include $(top_srcdir)/git.mk

src/hb-ucdn/README

+Contents of this directory are derived from UCDN:
+
+  https://github.com/grigorig/ucdn
+
+The original README follows:
+
+
 UCDN - Unicode Database and Normalization
 
 UCDN is a Unicode support library. Currently, it provides access