[chore] Add language to CONTRIBUTING re: pre-release spec features (#6160)
[chore] Add language to CONTRIBUTING re: pre-release spec features (#6160)

Signed-off-by: Anthony J Mirabella <bb65d5af60f06ca654295bbce73e2d43310f732e@aneurysm9.com>

open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector