Document the new emscripten-specific build option.

Docs/GettingStarted.dox

@@ -168,6 +168,7 @@ A number of build options can be defined when invoking the build scripts or when
 |EMSCRIPTEN_ROOT_PATH |-|Root path to Emscripten cross-compiler tools (Emscripten only)|
 |EMSCRIPTEN_SYSROOT   |-|Path to Emscripten system root (Emscripten only)|
 |EMSCRIPTEN_WASM      |1|Enable Binaryen support to generate output to WASM (WebAssembly) format (Emscripten only)|
+|EMSCRIPTEN_AUTO_SHELL|1|Auto adding a default HTML shell-file when it is not explicitly specified (Emscripten only)|
 |EMSCRIPTEN_ALLOW_MEMORY_GROWTH|*|Enable memory growing based on application demand when targeting asm.js, it is not set by default due to performance penalty (Emscripten with STATIC or SHARED library type only)|
 |EMSCRIPTEN_TOTAL_MEMORY|*|Specify the total size of memory to be used (Emscripten only); default to 128 MB, must be in multiple of 64 KB when targeting WebAssembly and in multiple of 16 MB when targeting asm.js|
 |EMSCRIPTEN_SHARE_DATA|0|Enable sharing data file support (Emscripten only)|