[tracing] Add trace event best practises. This CL adds a readme to guide devs to best practises when emitting trace events in chrome. Change-Id: I0b59a6acd81feaae35984bf6dd97dac1b99d76bf Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6709793 Reviewed-by: Eric Seckler <[email protected]> Reviewed-by: Mikhail Khokhlov <[email protected]> Commit-Queue: Etienne Pierre-Doray <[email protected]> Cr-Commit-Position: refs/heads/main@{#1485126} NOKEYCHECK=True GitOrigin-RevId: 52e9c708755f0a409a2755f39b62ad496998a07b