[editorial] Ensure every section has a README

[chalin]

Originally posted by @tigrannajaryan in https://github.com/open-telemetry/opentelemetry-specification/issues/2479#issuecomment-1092923011

every section will need a README. I'm available to help here: e.g., I submit a first PR that adds a README (with only a title) to each section that doesn't have such a file. That'll act as a placeholder that can be built upon via subsequent PRs. WDYT?

Yes, sounds good. Some sub-directories have good candidates for README (e.g. most that have an "overview.md" should work as README).

• [x] #2479
• [x] #2559
• [x] #2625
• [x] #2629