readme tweak

README.md

@@ -22,6 +22,12 @@ This module provides a function to handle this validation.
 
 ### usage
 
+arguments
+* `cert_url`  full url of the certificate to verify (from HTTP request header named `signaturecertchainurl`)
+* `signature` signature of the request (from HTTP request header named `signature`)
+* `requestRawBody`  full body string from POST request
+* `callback`  (optional) completion function. has 1 argument which indicates error. falsey when verification passes
+
 You may include a callback function, in the standard node error argument-first format:
 
 ```javascript
@@ -40,9 +46,3 @@ const verifier = require('alexa-verifier')
 
 const verifyPromise = verifier(cert_url, signature, requestRawBody)
 ```
-
-
-* `cert_url`  full url of the certificate to verify (from HTTP request header named `signaturecertchainurl`)
-* `signature` signature of the request (from HTTP request header named `signature`)
-* `requestRawBody`  full body string from POST request
-* `callback`  completion function. has 1 argument which indicates error. falsey when verification passes