Set up shields.io badges

[hobofan]

Badges in READMEs can be a really good way to improve the developer experience for an ecosystem, as they can provide a consistently recognizable link to a website with more information (like the registry) in an otherwise unstructured (= of always varying structure) block of text.

Shields.io provides the commonly used service for that (with integrations to all the popular package managers): https://shields.io/category/version

Examples for what the badge could look like:

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This is a twin issue to bazel-contrib/bcr-ui#29 which I want to tackle at some point. Depending on where I get around to implementing it first, I think we can benefit from the experience.