feat: add instructions to open terminal with watcher report to README.md

[mbresson]

Hi, nice project! I was looking for a simple screen time / activity watcher (tried ActivityWatch which is great but recorded too much information and uploads files to a remote bucket which is a big no for me).

I updated your README for other people like me who want to open a terminal window with the content of today's activity refreshing automatically. Feel free to merge it (or close it if you think that's not useful). (also fixed a typo if you don't mind)