Electrum.js specified what is supported for now

lib/Electrum.js

@@ -7,6 +7,9 @@ var Key = require('./Key'),
 /**
  * Pre-BIP32 Electrum public key derivation (electrum <2.0)
  *
+ * For now, this class can only understands master public keys.
+ * It doesn't support derivation from a private master key (TODO).
+ *
  * @example examples/ElectrumMPK.js
  */
 function Electrum (master_public_key) {