passkey : add short intro to README.md [no-ci] (#8317)

* passkey : add short intro to README.md [no-ci]

This commit adds a short introduction to the README.md file in the
examples/passkey directory.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* Update examples/passkey/README.md

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Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>