Changes to rand_core readme

[pitdicker]

I looked up the pre-release of rand_core on crates.io, and found some things that could improve in the readme.

Added badges, link to changelog, removed 'primarily' from licence text, added license links, and fixed broken link to Rand.

Example rendering: https://github.com/pitdicker/rand/blob/rand_core_readme/rand_core/README.md

[pitdicker]

@dhardy Forgot you also made changes to the readme of rand_core in #328 (not only rand). Do you feel like copying some of the changes from here?

[dhardy]

Why not just rebase this on top of my PR?

[pitdicker]

I will 😄.

[pitdicker]

Do we also want to document the minimum Rust version for rand-core?
Currently it oldest version it works with is 1.16.0, but we probably want to guarantee the same version as is the minimum for Rand and on the CI, Rust 1.22.

[dhardy]

You can add the minimum Rust version if you like (along with extra CI test), but I don't see much point; once for the whole Rand project should be enough since most people will use rand anyway.

[dhardy]

Looks good!

[dhardy]

Do we really need two doc links? Well, okay..

[pitdicker]

As there is only one version of rand-core that can be used with Rand right now, the minimum Rust version of rand-core doesn't matter. I think only when someone wants to start using a newer version of rand-core with an older version of Rand the minimum version becomes a thing. I'll leave things as they are for now.