document extended format
diff --git a/errors.go b/errors.go
index 8143650..4887263 100644
--- a/errors.go
+++ b/errors.go
@@ -43,6 +43,17 @@
// // unknown error
// }
//
+// Formatted printing of errors
+//
+// All error values returned from this package implement fmt.Formatter and can
+// be formatted by the fmt package. The following verbs are supported
+//
+// %s print the error. If the error has a Cause it will be
+// printed recursively
+// %v see %s
+// %+v extended format. Each Frame of the error's Stacktrace will
+// be printed in detail.
+//
// Retrieving the stack trace of an error or wrapper
//
// New, Errorf, Wrap, and Wrapf record a stack trace at the point they are
