better optout message for etelemetry

[satra]

we should add something like this to the etelemetry client.

Telemetry
---------
vcpkg collects usage data in order to help us improve your experience.
The data collected by Microsoft is anonymous.
You can opt-out of telemetry by re-running the bootstrap-vcpkg script with -disableMetrics,
passing --disable-metrics to vcpkg on the command line,
or by setting the VCPKG_DISABLE_METRICS environment variable.

Read more about vcpkg telemetry at docs/about/privacy.md