style guide: Describe Envoy naming convention for constants (#2273)
style guide: Describe Envoy naming convention for constants (#2273)

Calling out Envoy's constant naming convention and its difference from the Google style guide

Signed-off-by: Joshua Marantz <c3f6b7d9de5243f9f0884ee043fd42ff9df5f177@google.com>

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