README.md(s): Rust logo in crates.io badges

[tarcieri]

This is an issue to discuss whether we should put the Rust logo in the crates.io badges of our README.md files in the next release:

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I personally like it as it draws the eye to that badge in particular as one of the most important, and the logo is clearly visible for me.

[newpavlov]

Personally, I think it adds visual noise without substantial benefit. Rust users probably have no problem finding the badge and understanding its meaning since its use is ubiquitous in the Rust ecosystem. I also can not recall other popular Rust projects which include logo into crates.io badges.

[tarcieri]

adds visual noise

The badge is just as readable. If anything, I think it's a little more readable due to the added margins.