Added a doc for API

docs/api.markdown

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+# INTRO #
+
+The API is provided to query the data from a neaboard server by any client
+application.
+
+Tha data is returned in the json format and got by an http query.
+
+# METHODS #
+
+    /api/threads/N/?offset=M&tag=O
+
+Get a thread list. You will get N threads (required parameter) starting from
+Mth one (optional parameter, default is 0) with the tag O (optional parameter,
+threads with any tags are shown by default).
+
+    /api/tags/
+
+Get all active tag list. Active tag is a tag that has at least 1 active thread
+associated with it.
+
+    /api/thread/N/
+
+Get all Nth thread post. N is an opening post ID for the thread.
+
+In case of incorrect request you can get http error 404.