Multiple public keys code

Update README.md with the code example by @avimar (https://github.com/openpgpjs/openpgpjs/issues/271#issuecomment-274901731). Think that it's too hard to find and missed in documentation.

README.md

@@ -344,6 +344,16 @@ openpgp.generateKey(options).then(function(key) {
 });
 ```
 
+#### Encrypt with multiple public keys
+
+```js
+    publicKeys: [                      // for encryption
+	openpgp.key.readArmored(pubkey1).keys[0],
+	openpgp.key.readArmored(pubkey2).keys[0]  
+	]
+   };
+```
+
 #### Revoke a key
 
 Using a revocation signature: