build: Add GNUmakefile for header-only install purposes.

CMake isn't suited for this type of use, and this makes it so
that users don't have to manually copy headers around themselves.

GNUmakefile

@@ -0,0 +1,14 @@
+# Makefile
+
+all: default
+
+PREFIX=/usr/local
+
+install:
+	install -d $(PREFIX)/include/avisynth
+	cp avs_core/include/avisynth.h $(PREFIX)/include/avisynth
+	cp avs_core/include/avisynth_c.h $(PREFIX)/include/avisynth
+	cp -R avs_core/include/avs $(PREFIX)/include/avisynth
+
+uninstall:
+	rm -f -R $(PREFIX)/include/avisynth

README.md

@@ -6,3 +6,24 @@ features and developer friendliness.
 
 Visit our [homepage](http://avs-plus.net) for more information and binaries, or our
 [forum thread](http://forum.doom9.org/showthread.php?t=168856) for compilation instructions and support.
+
+
+Libav users:
+------------
+
+CMake isn't really suited to doing header-only installs, so a GNUmakefile is
+provided specifically for this purpose.
+
+### Using GNUmakefile:
+
+#### To install:
+
+>make install
+
+#### To install to a non-standard location:
+
+>make install PREFIX=/path/to/location
+
+#### To uninstall:
+
+>make uninstall