Readme: add repository info

[Drinausaur]

Add some info about releases and repos.

[halbux]

Thanks for this pr @Drinausaur. But to keep it simple I will just mention a general sentence that it is available in the official repos when it becomes available.

[Drinausaur]

That's fine.
However, for ubuntu, it is not in an official repo (yet, and we might have to wait a long time)
The PPA is not an official thing, but it's a way to get sparselizard without having to build it.

The icon code at the beginning generates a picture with the latest version and if you click on it, you'll see repositories where sparselizard is, for major distributions.I think it's worth it to keep it.

Edit : you can see a preview here :
https://github.com/Drinausaur/sparselizard/tree/readmeRepo