Consistent readme file for all components

[cijothomas]

https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-http has no link to crates.io, docs. https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-user-events-metrics has similar issue, has even less info than opentelemetry-http. https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-stdout has all the details, but the description could be improved.

Opening an issue to review the main readme.md file for each component, and make it consistent throughout.