Merge pull request #62 from adafruit/tyeth/issue57

Add message for HTTP response 422 to _errors.md

source/includes/_errors.md

@@ -9,6 +9,7 @@ Error Code | Meaning
 403 | Forbidden -- This action is not permitted.
 404 | Not Found -- The specified record could not be found.
 406 | Not Acceptable -- You requested a format that we don't serve.
+422 | Unprocessable Entity -- You've sent data we can't accept/understand. Is the value field (string/number) missing?
 429 | Too Many Requests -- You're sending or requesting data too quickly! Slow down!
 500 | Internal Server Error -- We had a problem with our server. Try again later.
 503 | Service Unavailable -- We're temporarily offline for maintenance. Please try again later.