Document the new conditional fastTimeout export (#2)

README.md

@@ -43,7 +43,7 @@ Alternatively, if you are directly using the library as a browser embed:
 
 ## Compatibility
 
-While `fastTimeout` is based on `postMessage`, it automatically falls back to `setImmediate` in environments where that function is defined. Hence, you can transparently use it in Node.js as well.
+While `fastTimeout` is based on `postMessage`, it automatically falls back to `setImmediate` in environments where that function is defined. Hence, you can transparently use it in Node.js as well. In environments where neither `postMessage` nor `setImmediate` is defined, it falls back to plain `setTimeout`; you get a performance degradation in that case, but at least your code will still work.
 
 ## Reference