add comment about use order to code example

Some people seem to miss the note (debitoor/chai-subset#69).  It's easy to forget about this too...I only really grokked this behavior after digging into what `chai-as-promise` does under the hood.

README.md

@@ -216,6 +216,7 @@ Do an `npm install chai-as-promised` to get up and running. Then:
 var chai = require("chai");
 var chaiAsPromised = require("chai-as-promised");
 
+// do this _after_ chai.use() for other plugins!
 chai.use(chaiAsPromised);
 
 // Then either: