Created POST api (markdown)

POST-api.md

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+## What do I send?
+
+There are two ways to send data up to become a paste on a haste-server:
+
+### Raw:
+
+__POST__: /documents
+__Content-type__: text/plain
+__Request body__: contents of paste
+__Response__: _See below_
+
+### Form Data:
+
+__POST__: /documents
+__Content-type__: multipart/form-data
+__Request body__: A multipart form data encoded body, with a single key named "data" which holds the paste
+__Response__: _See below_
+
+For more detail on the how multipart form bodies should be structured, see:
+http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4
+
+## What do I get back?
+
+A JSON document (content-type: `application/json`) containing a single Key-Value, ex:
+
+``` js
+{ "key": "as8fj2j2n" }
+```