add docs

docs/api/session.md

@@ -523,3 +523,25 @@ The `listener` will be called with `listener(details)` when an error occurs.
   * `timestamp` Double
   * `fromCache` Boolean
   * `error` String - The error description.
+
+#### `ses.protocol`
+
+Returns an instance of [protocol](protocol.md) module for this session.
+
+```javascript
+const electron = require('electron');
+const app = electron.app;
+const session = electron.session;
+const path = require('path');
+
+app.on('ready', function() {
+    const protocol = session.fromPartition(partitionName).protocol;
+    protocol.registerFileProtocol('atom', function(request, callback) {
+      var url = request.url.substr(7);
+      callback({path: path.normalize(__dirname + '/' + url)});
+    }, function (error) {
+      if (error)
+        console.error('Failed to register protocol')
+    });
+});
+```