Include explicit yarn version

To avoid confusion, specify an explicit version of Yarn and link to classic install docs.

README.md

@@ -44,7 +44,7 @@ Jellyfin Web is the frontend used for most of the clients available for end user
 
 ### Dependencies
 
-- Yarn
+- [Yarn 1.22.4](https://classic.yarnpkg.com/en/docs/install)
 - Gulp-cli
 
 ### Getting Started
@@ -78,4 +78,4 @@ Jellyfin Web is the frontend used for most of the clients available for end user
 
    ```sh
    yarn build:standalone
-   ```
+   ```