README: don't recommend go get

These days people will just import the packages and the go tool will
do the right thing. We don't need to explain it.

Add a pointer to the git repo, though.

For golang/go#62645

Change-Id: Ib6a6fb8989df8071b20d50de446d05b270c1f1ae
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/624195
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: ianlancetaylor

README.md

@@ -5,15 +5,6 @@
 
 oauth2 package contains a client implementation for OAuth 2.0 spec.
 
-## Installation
-
-~~~~
-go get golang.org/x/oauth2
-~~~~
-
-Or you can manually git clone the repository to
-`$(go env GOPATH)/src/golang.org/x/oauth2`.
-
 See pkg.go.dev for further documentation and examples.
 
 * [pkg.go.dev/golang.org/x/oauth2](https://pkg.go.dev/golang.org/x/oauth2)
@@ -33,7 +24,11 @@ The main issue tracker for the oauth2 repository is located at
 https://github.com/golang/oauth2/issues.
 
 This repository uses Gerrit for code changes. To learn how to submit changes to
-this repository, see https://golang.org/doc/contribute.html. In particular:
+this repository, see https://go.dev/doc/contribute.
+
+The git repository is https://go.googlesource.com/oauth2.
+
+Note:
 
 * Excluding trivial changes, all contributions should be connected to an existing issue.
 * API changes must go through the [change proposal process](https://go.dev/s/proposal-process) before they can be accepted.