Be more explicit about needing Yarn 1 (Classic) (#268)

* Be more explicit about needing Yarn 1 (Classic).

* Styling

Co-authored-by: Max Leiter <maxwell.leiter@gmail.com>

_docs/install-and-upgrade.md

@@ -95,7 +95,10 @@ Installing the [npm package](https://www.npmjs.com/package/thelounge) directly
 allows you to use stable releases on systems where we do not provide native
 packages.
 
-First, make sure latest [Node.js](https://nodejs.org/) LTS version (or latest current release) and [Yarn](https://legacy.yarnpkg.com/) are installed on your system. See [official instructions for Node.js](https://nodejs.org/en/download/package-manager/) and [official instructions for Yarn](https://legacy.yarnpkg.com/docs/install).
+First, make sure you have these installed on your system:
+
+- The latest [Node.js](https://nodejs.org/) LTS version (or latest current release). See the [official installation instructions](https://nodejs.org/en/download/package-manager/).
+- [Yarn 1 (classic)](https://legacy.yarnpkg.com/). See [official installation instructions](https://legacy.yarnpkg.com/docs/install).
 
 Then install The Lounge using: