Markdownify libwebp docs and reorganize them.

Break the main README into into multiple pages in the doc/ directory,
except for the tests, swig and webp_js docs which are in the corresponding
directories.
The webp mux doc is merged into the API doc and the tools doc.

Change-Id: Ia407617dd88094f4662841d37947cfef80799914

webp_js/README.md

@@ -0,0 +1,78 @@
+# Webp JavaScript decoder
+
+```
+ __   __ ____ ____ ____     __  ____
+/  \\/  \  _ \  _ \  _ \   (__)/  __\
+\       /  __/ _  \  __/   _)  \_   \
+ \__\__/_____/____/_/     /____/____/
+```
+
+This file describes the compilation of libwebp into a JavaScript decoder using
+Emscripten and CMake.
+
+-   install the Emscripten SDK following the procedure described at:
+    https://emscripten.org/docs/getting_started/downloads.html#installation-instructions-using-the-emsdk-recommended
+    After installation, you should have some global variable positioned to the
+    location of the SDK. In particular, $EMSDK should point to the top-level
+    directory containing Emscripten tools.
+
+-   configure the project 'WEBP_JS' with CMake using:
+
+    ```shell
+    cd webp_js && \
+    emcmake cmake -DWEBP_BUILD_WEBP_JS=ON \
+          ../
+    ```
+
+-   compile webp.js using 'emmake make'.
+
+-   that's it! Upon completion, you should have the webp.js and webp.wasm files
+    generated.
+
+The callable JavaScript function is WebPToSDL(), which decodes a raw WebP
+bitstream into a canvas. See webp_js/index.html for a simple usage sample (see
+below for instructions).
+
+## Demo HTML page
+
+The HTML page webp_js/index.html requires an HTTP server to serve the WebP image
+example. It's easy to just use Python for that.
+
+```shell
+cd webp_js && python -m SimpleHTTPServer 8080
+```
+
+and then navigate to http://localhost:8080 in your favorite browser.
+
+## Web-Assembly (WASM) version:
+
+CMakeLists.txt is configured to build the WASM version when using the option
+WEBP_BUILD_WEBP_JS=ON. The compilation step will assemble the files
+'webp_wasm.js', 'webp_wasm.wasm' in the webp_js/ directory. See
+webp_js/index_wasm.html for a simple demo page using the WASM version of the
+library.
+
+You will need a fairly recent version of Emscripten (at least 2.0.18,
+latest-upstream is recommended) and of your WASM-enabled browser to run this
+version.
+
+## Caveats
+
+-   First decoding using the library is usually slower, due to just-in-time
+    compilation.
+
+-   Some versions of llvm produce the following compile error when SSE2 is
+    enabled.
+
+    ```
+    "Unsupported:   %516 = bitcast <8 x i16> %481 to i128
+    LLVM ERROR: BitCast Instruction not yet supported for integer types larger than 64 bits"
+    ```
+
+    The corresponding Emscripten bug is at:
+    https://github.com/kripken/emscripten/issues/3788
+
+    Therefore, SSE2 optimization is currently disabled in CMakeLists.txt.
+
+-   If WEBP_ENABLE_SIMD is set to 1 the JavaScript version (webp.js) will be
+    disabled as wasm2js does not support SIMD.