README: add webpinfo section

Change-Id: Iee4a4ecbe562d6154f627ba62524cd1871a06564

README

@@ -113,8 +113,8 @@ make install
 
 CMake:
 ------
-With CMake, you can compile libwebp, cwebp, dwebp, gif2web, img2webp and the
-JS bindings.
+With CMake, you can compile libwebp, cwebp, dwebp, gif2web, img2webp, webpinfo
+and the JS bindings.
 
 Prerequisites:
 A compiler (e.g., gcc with autotools) and CMake.
@@ -367,6 +367,22 @@ Use following options to convert into alternate image formats:
   -quiet ....... quiet mode, don't print anything
   -noasm ....... disable all assembly optimizations
 
+WebP file analysis tool:
+========================
+
+'webpinfo' can be used to print out the chunk level structure and bitstream
+header information of WebP files. It can also check if the files are of valid
+WebP format.
+
+Usage: webpinfo [options] in_files
+Note: there could be multiple input files;
+      options must come before input files.
+Options:
+  -quiet ............. Do not show chunk parsing information.
+  -diag .............. Show parsing error diagnosis.
+  -summary ........... Show chunk stats summary.
+  -bitstream_info .... Parse bitstream header.
+
 Visualization tool:
 ===================