Update API-design-guidelines.md - code 422 added to table in 3.2

documentation/API-design-guidelines.md

@@ -333,6 +333,7 @@ Common errors are captured in the table below.
 | 409            | The 409 (Conflict) status code indicates when a request conflicts with the current state of the server.                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
 | 410            | The 410 (Gone) status code indicates that access to the target resource is no longer available at the origin server and that this condition is likely to be permanent.                                                                                                                                                                                                                                                                                                                                                                                                           |
 | 415            | The 415 (Unsupported Media Type) status code indicates that the server cannot accept the format of the request body as indicated by the `Content-Type` or `Content-Encoding` request header. The API specification will state what request body format should be used.                                                                                                                                                                                                                                                                                                           |
+| 422            | The 422 (Unprocessable Entity) status code means the server understands the content type of the request body, and the syntax of the request body is correct but it was unable to process the contained instructions.                                                                                                                                                                                                                                  |
 | 429            | The 429 (Too Many Requests) status code indicates that the server is temporarily unable to accept any more requests from the client due to the high number of requests recently sent. A `Retry-After` response header may indicate how long the client should wait before trying again.                                                                                                                                                                                                                                                                                          |
 | 500            | Status code 500 (Internal Server Error) indicates that the server encountered an unexpected condition that prevented it from fulfilling the request.<br>This code must always be documented. It should be used as a general system error.                                                                                                                                                                                                                                                                                                                                        |
 | 501            | Status code 501 (Not Implemented) indicates that the requested method is not supported by the server and cannot be handled. The only methods that servers require support (and therefore should not return this code) are `GET` and HEAD.                                                                                                                                                                                                                                                                                                                                        |