Mention tree traversal in package doc.

Readme.md

@@ -8,6 +8,7 @@
 This is errors,
 a drop-in replacement for [the standard Go errors package](https://pkg.go.dev/errors).
 It adds an API for wrapping an error with a caller-specified message and a snapshot of the call stack.
+It also adds a function for traversing an error’s tree of wrapped sub-errors.
 
 This module began as a fork of the venerable [github.com/pkg/errors](https://github.com/pkg/errors).
 That module’s GitHub repository has been frozen for a long time and has not kept up with changes to the standard library.

errors.go

@@ -1,5 +1,5 @@
 // Package errors is a drop-in replacement for the stdlib errors package.
-// It adds an error-wrapping API and stack traces.
+// It adds error-wrapping functions, stack traces, error-tree traversal.
 package errors
 
 import (