Add build instructions

codecs/visdif/BUILD.md

@@ -0,0 +1,14 @@
+This codec currently needs monkey-patching of Emscripten
+
+```
+$ docker run --rm -it -v $(PWD):/src squoosh-cpp "/bin/bash"
+# cat << EOF | patch /emsdk/upstream/emscripten/system/lib/dlmalloc.c
+659c659
+< #define MALLOC_ALIGNMENT ((size_t)(2 * sizeof(void *)))
+---
+> #define MALLOC_ALIGNMENT ((size_t)(16U))
+EOF
+# emcc --clear-cache
+# /emsdk/upstream/emscripten/embuilder build libdlmalloc --force
+# emmake make
+```