Document returning null from functions

README.md

@@ -42,20 +42,25 @@ or just download `ed2curve.js` or `ed2curve.min.js` and include it after
 Usage
 -----
 
-### ed2curve.convertKeyPair(keyPair)
+### ed2curve.convertKeyPair(keyPair) -> convertedKeyPair | null
 
 Converts the given key pair as generated by
 [TweetNaCl.js](https://github.com/dchest/tweetnacl-js)'s `nacl.sign.keyPair`
 into a key pair suitable for operations which accept key pairs generated by
 `nacl.box.keyPair`. This function is a combination of `convertPublicKey`
 and `convertSecretKey`.
 
-### ed2curve.convertPublicKey(edPublicKey)
+Returns `null` if the public key in the key pair is not a valid
+Ed25519 key.
+
+### ed2curve.convertPublicKey(edPublicKey) -> curvePublicKey | null
 
 Converts a 32-byte Ed25519 public key into a 32-byte Curve25519 public key
 and returns it.
 
-### ed2curve.convertSecretKey(edSecretKey)
+Returns `null` if the public key in not a valid Ed25519 key.
+
+### ed2curve.convertSecretKey(edSecretKey) -> curveSecretKey
 
 Converts a 64-byte Ed25519 secret key (or just the first 32-byte part of it,
 which is the secret value) into a 32-byte Curve25519 secret key and returns it.