all: update README

The previous description was outdated and incorrect.

Change-Id: Ib5fac87c354a558577d62c5785383eaffd0ed139
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/572375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/README.md b/README.md
index 43eda09..63972eb 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 # pkgsite-metrics
 
-This repository contains code that serves pkg.go.dev/metrics.
+This repository contains code that enables collecting and evaluating
+metrics for the Go ecosystem.
 
 ## Report Issues / Send Patches