Added sample document with post model reference

docs/dip-2.markdown

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+# 0 Title #
+
+"post" model reference
+
+# 1 Description #
+
+"post" is a model that defines an imageboard message, or post.
+
+# 2 Fields #
+
+# 2.1 Mandatory fields #
+
+* title -- text field.
+* text -- text field.
+* pub-time -- timestamp (TBD: Define format).
+
+# 2.2 Optional fields #
+
+* attachments -- defines attachments such as images.
+* attachment -- contains and attachment or link to attachment to be downloaded
+manually. Required attributes are mimetype and name.
+
+This field is used for the opening post (thread):
+
+* tags -- text tag name list.
+
+This field is used for non-opening post:
+
+* thread -- ID of a post model that this post is related to.