📇 Update README.md

[Snailedlt]

1. Update instructions for making and inviting your own Discordbot

The video link in the adding discordbot token section of the README.md is outdated.
Update it to be more clear and concise. Maybe also add a link to this helpful written guide.

2. Add links to install NodeJS and Yarn

1. Download NodeJS
2. Install Yarn
3. Verify installation

node --version
npm --version
yarn --version

Example output:

PS D:\code\splitscreen-me\splitscreenme-discordbot> node --version
v16.14.0
PS D:\code\splitscreen-me\splitscreenme-discordbot> npm --version
8.3.1
PS D:\code\splitscreen-me\splitscreenme-discordbot> yarn --version
1.22.11