Add instructions to the documentation how to get notified when there is a new release

[Sebastix]

Add a FAQ question: How do I get notified when there is a new release of Bitcoin-DCA?

Screen-Recording-2021-11-20-12-14-01.mp4

1. Click the watch button on the repository page on Github
2. Select Custom
3. Mark the checkbox 'releases' and click Apply

Now you will get a notification when there is a new Bitcoin-DCA release so you know you can upgrade your installation.