Updates protocol/know-your-http.md

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protocol/use-appropriate-status-codes.md

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 # Use Appropriate Status Codes
+Every API MUST use the appropriate [HTTP Status Codes](https://github.com/for-GET/know-your-http-well/blob/master/status-codes.md) to communicate the result of a request operation.
+
+Every API designer, implementer and consumer MUST understand the semantic of the HTTP Status Code she is using.
+
+At a minimum everyone MUST be familiar with the semantics of ["Common" HTTP Status Codes](https://github.com/for-GET/know-your-http-well/blob/master/status-codes.md#common).
+
+---
+
+#### Example
+A request: 
+
+```
+GET /order/1234 HTTP/1.1
+...
+```
+
+resulting in the **200 OK** response, when the requested resource (as identified by the request URI) couldn't be found: 
+
+```
+HTTP/1.1 200 OK
+Content-Type: application/json
+...
+
+
+{
+    "code": "NOT_FOUND_ERR_CODE"
+    "message" "Order 1234 wasn't found"
+}
+```
+is **illegal**.
+
+Instead the
+
+```
+HTTP/1.1 404 Not Found
+...
+```
+
+should be returned.