Development.md describes the wire-layer convention (one directory per
Onvif Web Service, gen_commands.py for new SOAP command types) but
does not mention that some directories also ship hand-written
higher-level helpers built on top of those types. A new contributor
reading the doc could reasonably assume event/ is purely
auto-generated and miss event/stream.
Adds a 'Higher-level helpers' section that calls out:
* event/stream — the new channel-based event consumer.
* event/topic — the existing topic identifier helpers.
Also documents the placement convention (sub-package under the
relevant web service directory) so future helpers land in a
predictable spot.
