add swig interface

Currently only supports a subset of decode functions and likely only
works fully for java.

For java bindings:
The generated java source can be compiled and the class files added to
libwebp.jar.
The generated jni source can be compiled to, e.g., libwebp_jni.so, which
can then be loaded via System.loadLibrary("webp_jni").

Change-Id: I8225933cbaf85c9cfa4b78c2e5f167cee8b62408

README

@@ -76,6 +76,26 @@ Note: The encoding and decoding libraries are compiled separately
 can be installed independently using a minor modifications in the
 corresponding Makefile.am configure files (see comments there).
 
+SWIG bindings:
+--------------
+
+To generate language bindings from swig/libwebp.i swig-1.3
+(http://www.swig.org) is required. 2.0 may work, but has not been tested.
+
+Currently from the decode interface the following functions are mapped:
+  WebPGetDecoderVersion
+  WebPGetInfo
+  WebPDecodeRGB
+  WebPDecodeRGBA
+  WebPDecodeBGR
+  WebPDecodeBGRA
+
+Java bindings:
+
+To build the swig-generated JNI wrapper code at least JDK-1.5 (or equivalent)
+is necessary for enum support. The output is intended to be a shared object /
+DLL that can be loaded via System.loadLibrary("webp_jni").
+
 Encoding tool:
 ==============