[ci skip] (docs) document include.through option for Belongs-To-Many relations.

Fixes #2975 and #3416

docs/docs/associations.md

@@ -258,6 +258,19 @@ UserProjects = sequelize.define('userProjects', {
   status: DataTypes.STRING
 })
 ```
+With Belongs-To-Many you can query based on **through** relation and select specific attributes. For example using `findAll` with **through**
+
+```js
+User.findAll({
+  include: [{
+    model: Project,
+      through: {
+        attributes: ['createdAt', 'startedAt', 'finishedAt']
+          where: {completed: true}
+      }
+  }]
+});
+```
 
 ## Scopes
 This section concerns association scopes. For a definition of association scopes vs. scopes on associated models, see [Scopes](scopes).