Add sources app metadata help text migration

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/sources/migrations/0021_auto_20190629_0648.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-06-29 06:48
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('sources', '0020_auto_20181128_0752'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='emailbasemodel',
+            name='metadata_attachment_name',
+            field=models.CharField(default='metadata.yaml', help_text='Name of the attachment that will contains the metadata type names and value pairs to be assigned to the rest of the downloaded attachments.', max_length=128, verbose_name='Metadata attachment name'),
+        ),
+    ]