Fixed #166 -- Highlight the significance of the double underscore syntax

django_models/README.md

@@ -118,6 +118,8 @@ Let's open `blog/models.py`, remove everything from it and write code like this:
         def __str__(self):
             return self.title
 
+> **Note** Double-check that you used two undescore characters (`_`) on each side of `str`. Those are used frequently in Python and sometimes we also call them "dunder" (short for "double-underscore").
+
 It is scary, right? But no worries, we will explain what these lines mean!
 
 All lines starting with `from` or `import` are lines that add some bits from other files. So instead of copying and pasting the same things in every file, we can include some parts with `from ... import ...`.

django_orm/README.md

@@ -99,6 +99,8 @@ Or maybe we want to see all the posts that contain a word 'title' in the `title`
     >>> Post.objects.filter(title__contains='title')
     [<Post: Sample title>, <Post: 4th title of post>]
 
+> **Note** There are two underscore characters (`_`) between `title` and `contains`. Django's ORM uses this syntax to separate field names ("title") and operations or filters ("contains"). If you only use one underscore, you'll get an error like "FieldError: Cannot resolve keyword title_contains".
+
 You can also get a list of all published posts. We do it by filtering all the posts that have `published_date`:
 
     >>> Post.objects.filter(published_date__isnull=False)