Add quick overview about addresses to doc.go.

Closes #4.

doc.go

@@ -18,5 +18,29 @@ A Tx defines a bitcoin transaction that provides more efficient manipulation of
 raw wire protocol transactions.  It memoizes the hash for the transaction on its
 first access so subsequent accesses don't have to repeat the relatively
 expensive hashing operations.
+
+Address Overview
+
+The Address interface provides an abstraction for a Bitcoin address.  While the
+most common type is a pay-to-pubkey-hash, Bitcoin already supports others and
+may well support more in the future.  This package currently provides
+implementations for the pay-to-pubkey, pay-to-pubkey-hash, and
+pay-to-script-hash address types.
+
+To decode/encode an address:
+
+	// NOTE: The default network is only used for address types which do not
+	// already contain that information.  At this time, that is only
+	// pay-to-pubkey addresses.
+	addrString := "04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962" +
+		"e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d57" +
+		"8a4c702b6bf11d5f"
+	defaultNet := &btcnet.MainNetParams
+	addr, err := btcutil.DecodeAddress(addrString, defaultNet)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(addr.EncodeAddress())
 */
 package btcutil