add info on public key recovery

lib/ecdsa.js

@@ -57,6 +57,9 @@ ECDSA.prototype.randomK = function() {
   return this;
 };
 
+// Information about public key recovery:
+// https://bitcointalk.org/index.php?topic=6430.0
+// http://stackoverflow.com/questions/19665491/how-do-i-get-an-ecdsa-public-key-from-just-a-bitcoin-signature-sec1-4-1-6-k
 ECDSA.prototype.sig2pubkey = function() {
   var i = this.sig.i;
   if (!(i === 0 || i === 1 || i === 2 || i === 3))