Update README with new Cache Provider interface (node-saml#608)

Cache Provider interface has changed in node-saml#532 . This updates the readme for the new interface.

Author: cybercase

README.md

@@ -359,28 +359,19 @@ To support this scenario you can provide an implementation for a cache provider
 
 ```javascript
 {
-    save: function(key, value, callback) {
-      // save the key with the optional value, invokes the callback with the value saves
+    saveAsync: async function(key, value) {
+      // saves the key with the optional value, returns the saved value
     },
-    get: function(key, callback) {
-      // invokes 'callback' and passes the value if found, null otherwise
+    getAsync: async function(key) {
+      // returns the value if found, null otherwise
     },
-    remove: function(key, callback) {
-      // removes the key from the cache, invokes `callback` with the
+    removeAsync: async function(key) {
+      // removes the key from the cache, returns the
       // key removed, null if no key is removed
     }
 }
 ```
 
-The `callback` argument is a function in the style of normal Node callbacks:
-
-```
-function callback(err, result)
-{
-
-}
-```
-
 Provide an instance of an object which has these functions passed to the `cacheProvider` config option when using Passport-SAML.
 
 ## SLO (single logout)