Update Metrics Semantic Conventions README

[justinfoote]

We have several open PRs related to semantic conventions:

• https://github.com/open-telemetry/opentelemetry-specification/pull/937
• https://github.com/open-telemetry/opentelemetry-specification/pull/1076
• https://github.com/open-telemetry/opentelemetry-specification/pull/1077
• https://github.com/open-telemetry/opentelemetry-specification/pull/1052

Only one of them is attempting to update the metrics semantic conventions README.

This issue is a placeholder for us to sort out the readme once all these PRs have resolved. This issue will be fixed once all the metrics semantic conventions have merged or closed and the readme has been updated to be coherent, and to include links to all the child semantic conventions.

[justinfoote]

Also, potentially update the general timing convention (assuming it gets merged). See: https://github.com/open-telemetry/opentelemetry-specification/pull/1142#discussion_r517762323