README.md: change the build instruction

After go1.16, `go get` runs in modules mode, and it's not the
recommended way of installing binaries nor a way to fetch source
code. Given that this example repo is designed for new go users
who want to learn, it is better to guide users to clone the repo
using the git command.

Updated the instruction for appengine-hello since goapp is gone.
I didn't test the appengine-hello instruction myself and not sure
if it still runs.

Change-Id: I74a829203def9b062bee75637c7d2c6cef448a2f
Reviewed-on: https://go-review.googlesource.com/c/example/+/340113
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

README.md

@@ -1,15 +1,24 @@
 # Go example projects
 
-
 [![Go Reference](https://pkg.go.dev/badge/golang.org/x/example.svg)](https://pkg.go.dev/golang.org/x/example)
 
 This repository contains a collection of Go programs and libraries that
 demonstrate the language, standard libraries, and tools.
 
-## [hello](hello/) and [stringutil](stringutil/)
+## Clone the project
 
-    go get golang.org/x/example/hello
+```
+$ git clone https://go.googlesource.com/example
+$ cd example
+```
+https://go.googlesource.com/example is the canonical Git repository.
+It is mirrored at https://github.com/golang/example.
+## [hello](hello/) and [stringutil](stringutil/)
 
+```
+$ cd hello
+$ go build
+```
 A trivial "Hello, world" program that uses a stringutil package.
 
 Command [hello](hello/) covers:
@@ -26,8 +35,10 @@ Library [stringutil](stringutil/) covers:
 
 ## [outyet](outyet/)
 
-    go get golang.org/x/example/outyet
-
+```
+$ cd outyet
+$ go build
+```
 A web server that answers the question: "Is Go 1.x out yet?"
 
 Topics covered:
@@ -45,12 +56,10 @@ Topics covered:
 
 ## [appengine-hello](appengine-hello/)
 
-	goapp get golang.org/x/example/appengine-hello
-
 A trivial "Hello, world" App Engine application intended to be used as the
-starting point for your own code.
-
-_Note_: The `goapp` tool is part of the [Google App Engine SDK for Go](https://cloud.google.com/appengine/downloads#Google_App_Engine_SDK_for_Go).
+starting point for your own code. Please see
+[Google App Engine SDK for Go](https://cloud.google.com/appengine/downloads#Google_App_Engine_SDK_for_Go)
+and [Quickstart for Go in the App Engine Standard Environment](https://cloud.google.com/appengine/docs/standard/go/quickstart).
 
 ## [gotypes](gotypes/)