Updated document

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).
+* version -- when post content changes, version should be incremented.
 
 # 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.
 * tripcode