add install links

Author: leo60228

README.md

@@ -6,8 +6,8 @@ Building is done via scripts defined in the package.json.
 ### Requirements
 Development was done on NixOS 20.09. A `flake.nix` is provided, allowing for replication of the exact build environment. Alternatively, the following versions can be installed:
 
-* Node.js 14.9.0
-* Yarn 1.22.5
+* Node.js 14.9.0 ([downloads](https://nodejs.org/download/release/v14.9.0/), [installation](https://github.com/nodejs/help/wiki/Installation))
+* Yarn 1.22.5 ([installation](https://classic.yarnpkg.com/en/docs/install/))
 
 ### Instructions
 Packages can be installed via `yarn`. After this, the following (relevant) scripts are available via `yarn run <script>`: