Add an example that reproduces a pending issue

This should help make testing easier and should server as a reference
for something we'll want to write a test for in future.

example/pexp/migrations/0002_modelc_field4.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9b1 on 2015-10-24 01:42
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pexp', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='modelc',
+            name='field4',
+            field=models.ManyToManyField(related_name='related_c', to='pexp.ModelB'),
+        ),
+    ]

example/pexp/models.py

@@ -19,6 +19,7 @@ class ModelB(ModelA):
     field2 = models.CharField(max_length=10)
 class ModelC(ModelB):
     field3 = models.CharField(max_length=10)
+    field4 = models.ManyToManyField(ModelB, related_name='related_c')
 
 class nModelA(models.Model):
     field1 = models.CharField(max_length=10)