docs: add readme badges

[sonro]

Add these badges to the readme:

• 
• 
• 
• 

Are there any others we should want?

[allan2]

MSRV would be nice. We could replace the sentence in the opening block with a badge.

[Dylan-DPC]

I usually prefer not to include the license badge because it is available elsewhere (such as the sidebar in github or elsewhere or thrown in the menu somewhere :P ) and the fact that github api doesn't support multiple licenses (and I just realised that this repo is only MIT licensed for some reason which is rare but fine i guess :P )

I would recommend adding a msrv badge (you would need a custom badge for this) since it is a thing many people search for in a repo and having in on top would make things easier

[sonro]

• MSRV added
• License removed

[github-actions[bot]]

[allan2]

@sonro is already familiar with MSRV badges. I copied his on my latest repo ;)

Thanks!