docs(model): add blog post link and example to syncIndexes() docs

lib/model.js

@@ -1027,6 +1027,17 @@ Model.init = function init(callback) {
  * schema. This function will drop any indexes that are not defined in
  * the model's schema except the `_id` index, and build any indexes that
  * are in your schema but not in MongoDB.
+ * 
+ * See the [introductory blog post](http://thecodebarbarian.com/whats-new-in-mongoose-5-2-syncindexes)
+ * for more information.
+ * 
+ * ####Example:
+ * 
+ *     const schema = new Schema({ name: { type: String, unique: true } });
+ *     const Customer = mongoose.model('Customer', schema);
+ *     await Customer.createIndex({ age: 1 }); // Index is not in schema
+ *     // Will drop the 'age' index and create an index on `name`
+ *     await Customer.syncIndexes();
  *
  * @param {Object} [options] options to pass to `ensureIndexes()`
  * @param {Function} [callback] optional callback