README: update for perf and perfdata packages

Add these packages to the list in the top-level README.md and copy the
explanation of how to run a local testing instance to perf/README.md.

For golang/go#48803.

Change-Id: Ie25ef518e8df1403843e0591bdb3b7ab9b6463c9
Reviewed-on: https://go-review.googlesource.com/c/build/+/397481
Trust: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

README.md

@@ -122,6 +122,19 @@ The main components of the Go build system are:
     dashboard
 
 
+* The **perfdata** server, in
+  [perfdata/appengine](https://dev.golang.org/dir/build/perfdata/appengine)
+  serves https://perfdata.golang.org/. It runs on App Engine and
+  serves the benchmark result storage system.
+
+
+* The **perf** server, in
+  [perf/appengine](https://dev.golang.org/dir/build/perf/appengine)
+  serves https://perf.golang.org/. It runs on App Engine and serves
+  the benchmark result analysis system. See [its
+  README](perf/README.md) for how to start a local testing instance.
+
+
 ### Adding a Go Builder
 
 If you wish to run a Go builder, please email

perf/README.md

@@ -0,0 +1,18 @@
+This package contains the https://perf.golang.org/ benchmark result
+analysis system. It serves as a front-end to the benchmark result
+storage system at https://perfdata.golang.org/.
+
+Both storage and analysis can be run locally; the following commands will run
+the complete stack on your machine with an in-memory datastore.
+
+```
+go install golang.org/x/build/perfdata/localperfdata@latest
+go install golang.org/x/build/perf/localperf@latest
+localperfdata -addr=:8081 -view_url_base=http://localhost:8080/search?q=upload: &
+localperf -addr=:8080 -storage=http://localhost:8081
+```
+
+The storage system is designed to have a standardized REST
+API at https://perfdata.golang.org/, and we encourage additional analysis
+tools to be written against the API. An example client can be found in the
+[perfdata](https://pkg.go.dev/golang.org/x/build/perfdata) package.