Update token documentation (adding expiresIn)

Author: blakeembrey

README.md

@@ -54,10 +54,15 @@ To re-create an access token instance and make requests on behalf on the user, y
 // Can also just pass the raw `data` object in place of an argument.
 var token = githubAuth.createToken('access token', 'optional refresh token', 'optional token type', { data: 'raw user data' })
 
-// Refresh the users credentials and save the updated access token.
+// Set the token TTL.
+token.expiresIn(1234) // Seconds.
+token.expiresIn(new Date('2016-11-08')) // Date.
+
+// Refresh the users credentials and save the new access token and info.
 token.refresh().then(storeNewToken)
 
-// Sign a standard HTTP request object, updating URL with token or authorization headers.
+// Sign a standard HTTP request object, updating the URL with the access token
+// or adding authorization headers, depending on token type.
 token.sign({
   method: 'get',
   url: 'https://api.github.com/users'