Updated readme with HTTP info

errors/README.md

@@ -59,6 +59,30 @@ if err := db.Select(query); err != nil {
 }
 ```
 
+## func HTTP()
+Create an HTTP error, which is the same as others but includes and HTTP Status code:
+```
+if err := db.Select(query); err != nil {
+    return errors.HTTP(http.StatusNotFound, err, "cannot read users")
+}
+```
+
+To retrieve the code, use err.Code() which returns integer value.
+
+If you wrapped an HTTP error in another HTTP error, there may be different codes in the stack.
+The Code() method will dig down all the causes and return the lowest non-zero code value it finds.
+So StatusNotFound (404) will have precedence over StatusInsufficientStorage (507)
+It will return 0 when there are no codes in the error stack.
+
+An error with HTTP code will also print the code in the stack, e.g.:
+```
+    http-error_test.go:27: failed:
+        	http-error_test.go(24): TestHTTPError() terrible mistake HTTP(404:Not Found), because
+        	http-error_test.go(21): TestHTTPError() jissis this is bad! HTTP(507:Insufficient Storage), because
+        	http-error_test.go(18): TestHTTPError() failed to get user HTTP(400:Bad Request), because
+        	http-error_test.go(14): TestHTTPError() failed to connect to db
+```
+
 ## Refactoring exiting code
 Replace all other errors package imports with this package:
 ```