Adding build readme to WebP plugin

Former-commit-id: 0db0b497ec6f9c99fbd2ba3bddaa0afd5c565ccb
Former-commit-id: bc9c441cef8f9a9d58a66d42f653a7587665ffe7

src/Plugins/ImageProcessor/ImageProcessor.Plugins.WebP/README.md

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+#Build instructions for libwebp.dll 
+
+Download libwebp source via git or through http download.
+
+In Start Menu, run Visual Studio Tools > Command Prompt.
+
+Change to the libwebp directory and run 
+
+    nmake /f Makefile.vc CFG=release-dynamic RTLIBCFG=dynamic OBJDIR=output
+
+Repeat with the
+Visual Studio x64 Cross Tools Command Prompt
+
+
+Copy to x86 and x64 directories from /output/
+
+To verify p/invokes have not changed:
+
+Review the following history logs for changes since the last release:
+
+http://git.chromium.org/gitweb/?p=webm/libwebp.git;a=history;f=src/webp/types.h;hb=HEAD
+http://git.chromium.org/gitweb/?p=webm/libwebp.git;a=history;f=src/webp/encode.h;hb=HEAD
+http://git.chromium.org/gitweb/?p=webm/libwebp.git;a=history;f=src/webp/decode.h;hb=HEAD