README: add a section on gif2webp

Change-Id: Ie701da16223df7482f7b017150242ba2d381d0b5

README

@@ -324,6 +324,43 @@ $ make -f makefile.unix examples/vwebp
 > nmake /f Makefile.vc CFG=release-static \
     ../obj/x64/release-static/bin/vwebp.exe
 
+Animated GIF conversion:
+========================
+Animated GIF files can be converted to WebP files with animation using the
+gif2webp utility available under examples/. The files can then be viewed using
+vwebp.
+
+Usage:
+ gif2webp [options] gif_file -o webp_file
+options:
+  -h / -help  ............ this help
+  -lossy ................. Encode image using lossy compression.
+  -mixed ................. For each frame in the image, pick lossy
+                           or lossless compression heuristically.
+  -q <float> ............. quality factor (0:small..100:big)
+  -m <int> ............... compression method (0=fast, 6=slowest)
+  -kmin <int> ............ Min distance between key frames
+  -kmax <int> ............ Max distance between key frames
+  -f <int> ............... filter strength (0=off..100)
+  -metadata <string> ..... comma separated list of metadata to
+                           copy from the input to the output if present.
+                           Valid values: all, none, icc, xmp (default)
+  -mt .................... use multi-threading if available
+
+  -version ............... print version number and exit.
+  -v ..................... verbose.
+  -quiet ................. don't print anything.
+
+Building:
+---------
+With the libgif development files installed, gif2webp can be built using
+makefile.unix:
+$ make -f makefile.unix examples/gif2webp
+
+or using autoconf:
+$ ./configure --enable-everything
+$ make
+
 Encoding API:
 =============