Add comments explaining include paths

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/Makefile b/library/Makefile
index 801304b..20a5984 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -5,6 +5,10 @@
 WARNING_CFLAGS ?=  -Wall -Wextra
 LDFLAGS ?=
 
+# Include ../include for public headers and . for private headers.
+# Note that . needs to be included explicitly for the sake of library
+# files that are not in the /library directory (which currently means
+# under /3rdparty).
 LOCAL_CFLAGS = $(WARNING_CFLAGS) -I. -I../include -D_FILE_OFFSET_BITS=64
 LOCAL_LDFLAGS =