Add rationale.md to document design decisions that people may be curi… (#1627)
Add rationale.md to document design decisions that people may be curi… (#1627)

* Add rationale.md to document design decisions that people may be curious about later.

* Formatting

* OT -> OTel
open-telemetry/opentelemetry-java,open-telemetry/opentelemetry-java,open-telemetry/opentelemetry-java