Begun GridFS documentation

docs/guide/gridfs.rst

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+======
+GridFS
+======
+GridFS support comes in the form of the :class:`~mongoengine.FileField` field
+object. This field acts as a file-like object and provides a couple of
+different ways of inserting and retrieving data. Metadata such as content-type
+can also be stored alongside the stored files. In the following example, an
+document is created to store details about animals, including a photo:
+
+    class Animal(Document):
+        genus = StringField()
+        family = StringField()
+        photo = FileField()
+
+    marmot = Animal('Marmota', 'Sciuridae')
+
+    marmot_photo = open('marmot.jpg')   # Retrieve a photo from disk
+    marmot.photo = marmot_photo         # Store the photo in the document
+
+    marmot.save()
+
+So adding file data to a document is as easy as adding data to any other 
+
+.. versionadded:: 0.4
+
+