Add the available routes to the README of zPages (#3132)
Add the available routes to the README of zPages (#3132)

Added the available routes in the zPages extension README for the users
convenience.
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector