Mention WebAssembly in README, fixes dcodeIO#51

README.md

@@ -229,6 +229,11 @@ API
 * Long#**xor**(other: `Long | number | string`): `Long`<br />
   Returns the bitwise XOR of this Long and the given one.
 
+WebAssembly support
+-------------------
+
+[WebAssembly](http://webassembly.org) supports 64-bit integer arithmetic out of the box, hence a [tiny WebAssembly module](./src/wasm.wast) is used to compute operations like multiplication, division and remainder more efficiently (slow operations like division are around twice as fast), falling back to floating point based computations in JavaScript where WebAssembly is not yet supported, e.g., in older versions of node.
+
 Building
 --------