[nongnu] elpa/buttercup 6372fd46b2: Update README.md with NonGNU ELPA badge and installation

[ELPA Syncer]

branch: elpa/buttercup
commit 6372fd46b20cc807157071cb08ad263132ce7339
Author: Ola Nilsson <ola.nilsson@gmail.com>
Commit: Ola Nilsson <ola.nilsson@gmail.com>

    Update README.md with NonGNU ELPA badge and installation
---
 README.md | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/README.md b/README.md
index 4bb1e5d6b1..050e193c2f 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 # Buttercup — Behavior-Driven Emacs Lisp Testing
 
 [![Build and 
test](https://github.com/jorgenschaefer/emacs-buttercup/workflows/Build%20and%20test/badge.svg)](https://github.com/jorgenschaefer/emacs-buttercup/actions?query=workflow%3A%22Build+and+test%22+branch%3Amaster)
+[![NonGNU 
ELPA](https://elpa.nongnu.org/nongnu/buttercup.svg)](https://elpa.nongnu.org/nongnu/buttercup.html)
 [![MELPA 
Stable](http://stable.melpa.org/packages/buttercup-badge.svg)](http://stable.melpa.org/#/buttercup)
 
 ![Ranculus repens, photo by sannse](docs/images/buttercup.jpg)
@@ -31,11 +32,15 @@ A simple test looks like this.
 *Full article: [Running Tests](docs/running-tests.md)*
 
 You can install buttercup from
+[NonGNU ELPA](https://elpa.nongnu.org/) or
 [MELPA Stable](http://stable.melpa.org/). Add the following to your
 `init.el` or `.emacs` file:
 
 ```
 (require 'package)
+;; Available as a default in GNU Emacs from version 28.1
+(add-to-list 'package-archives
+             ("nongnu" . "https://elpa.nongnu.org/nongnu/";) t)
 (add-to-list 'package-archives
              '("melpa-stable" . "http://stable.melpa.org/packages/";) t)
 ```