Improve README for twitter plugin (#22786)

* Improve README for twitter plugin

* Update packages/gatsby-plugin-twitter/README.md

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

* Update packages/gatsby-plugin-twitter/README.md

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

* Update packages/gatsby-plugin-twitter/README.md

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

Co-authored-by: Laurie Barth <laurie@LauriesrkLaptop.fios-router.home>
Co-authored-by: Aisha Blake <aisha@gatsbyjs.com>

packages/gatsby-plugin-twitter/README.md

@@ -3,16 +3,22 @@
 Loads the Twitter JavaScript for embedding tweets, timelines, share and follow
 buttons. Lets you add tweets to markdown and in other places.
 
-Note: when copying the embed code, just copy the blockquote section and not the
-script.
-
 ## Install
 
-`npm install --save gatsby-plugin-twitter`
+```shell
+npm install gatsby-plugin-twitter
+```
 
-## How to use
+## Configure
 
 ```javascript
 // In your gatsby-config.js
 plugins: [`gatsby-plugin-twitter`]
 ```
+
+## How to Use
+
+1. On Twitter, find the tweet you'd like to display.
+2. In the top right-hand corner of the tweet, click the caret (∨) and select "Embed tweet".
+3. Take the code you're given and copy it into your markdown page.
+   > Note: when copying the embed code, copy **only** the blockquote section and not the script.