Link up nilerror from the main README (#28)

Just makes it easier to catalog what's in this repository and get a
quick explanation for `nilerror` without having to click all the way
into it.

Author: brandur

docs/README.md

@@ -5,4 +5,5 @@
 See:
 
 * [`datadogriver`](../datadogriver): Package containing examples of using `otelriver` with [DataDog](https://www.datadoghq.com/).
+* [`nilerror`](../nilerror): Package containing a River hook for detecting a common accidental Go problem where a nil struct value is wrapped in a non-nil interface value.
 * [`otelriver`](../otelriver): Package for use with [OpenTelemetry](https://opentelemetry.io/).

nilerror/README.md

@@ -1,6 +1,16 @@
 # nilerror [![Build Status](https://github.com/riverqueue/rivercontrib/actions/workflows/ci.yaml/badge.svg?branch=master)](https://github.com/riverqueue/rivercontrib/actions) [![Go Reference](https://pkg.go.dev/badge/github.com/riverqueue/rivercontrib.svg)](https://pkg.go.dev/github.com/riverqueue/rivercontrib/nilerror)
 
-Provides a River hook for detecting a common Go error where a nil struct value is wrapped in a non-nil interface value. This commonly causes trouble with the error interface, where an unintentional nil error is returned.
+Provides a River hook for detecting a common accidental Go problem where a nil struct value is wrapped in a non-nil interface value. This commonly causes trouble with the error interface, where an unintentional non-nil error is returned. For example:
+
+``` go
+func returnsError() error {
+    var p *MyError = nil
+    if bad() {
+        p = ErrBad
+    }
+    return p // Will always return a non-nil error.
+}
+```
 
 See https://go.dev/doc/faq#nil_error.