Add a README to telemetry/internal/bin/
Add a README to telemetry/internal/bin/

Files should not be checked into this folder, as it is reserved for use with
the BinaryManager.

BUG=538738

Review URL: https://codereview.chromium.org/1380843003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352216}

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