README.md: make pkgsite more discoverable

And update the installation instruction.

Change-Id: I950f2a990ccddbc775b59b51b2b667ab40977237
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/509416
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Bypass: Hyang-Ah Hana Kim <hyangah@gmail.com>

README.md

@@ -1,13 +1,30 @@
-# Pkg.go.dev
+# golang.org/x/pkgsite
+
+This repository hosts the source code of the [pkg.go.dev](https://pkg.go.dev) website,
+and [`pkgsite`](https://pkg.go.dev/golang.org/x/pkgsite/cmd/pkgsite), a documentation
+server program.
 
 [![Go Reference](https://pkg.go.dev/badge/golang.org/x/pkgsite.svg)](https://pkg.go.dev/golang.org/x/pkgsite)
 
-## A site for discovering Go packages
+## pkg.go.dev: a site for discovering Go packages
 
 Pkg.go.dev is a website for discovering and evaluating Go packages and modules.
 
 You can check it out at [https://pkg.go.dev](https://pkg.go.dev).
 
+## pkgsite: a documentation server
+
+`pkgsite` program extracts and generates documentation for Go projects.
+
+Example usage:
+```
+$ go install golang.org/x/pkgsite/cmd/pkgsite@latest
+$ cd myproject
+$ pkgsite -open .
+```
+
+For more information, see the [pkgsite documentation](https://pkg.go.dev/golang.org/x/pkgsite/cmd/pkgsite).
+
 ## Requirements
 
 Pkgsite requires Go 1.19 to run.

cmd/pkgsite/main.go

@@ -6,15 +6,15 @@
 // It runs as a web server and presents the documentation as a
 // web page.
 //
-// To install, run `go install ./cmd/pkgsite` from the pkgsite repo root.
+// To install, run `go install golang.org/x/pkgsite/cmd/pkgsite@latest`.
 //
 // With no arguments, pkgsite will serve docs for main modules relative to the
 // current directory, i.e. the modules listed by `go list -m`. This is
 // typically the module defined by the nearest go.mod file in a parent
 // directory. However, this may include multiple main modules when using a
 // go.work file to define a [workspace].
 //
-// For example, both of the following the following forms could be used to work
+// For example, both of the following forms could be used to work
 // on the module defined in repos/cue/go.mod:
 //
 // The single module form: