Include v3 installation instructions alongside v4 (#388)

* Include link to v3 installation instructions

Helpful for us on older projects and installing this plugin.

* Add instructions instead of linking

* Update

* update

* Add code fence blocks

---------

Co-authored-by: Jordan Pittman <[email protected]>

Author: morrislaptop

README.md

@@ -33,6 +33,21 @@ Then add the plugin to your main `style.css` file:
 + @plugin "@tailwindcss/typography";
 ```
 
+If you are still using **Tailwind CSS v3**, add the plugin to your `tailwind.config.js` file:
+
+```
+// tailwind.config.js
+module.exports = {
+  theme: {
+    // ...
+  },
+  plugins: [
+    require('@tailwindcss/typography'),
+    // ...
+  ],
+}
+```
+
 ---
 
 ## Basic usage