cync_lan (Home Assistant custom_component) v2.4.2

Corrects the installation instructions. Earlier versions told you HACS
installs would not work, because the integration lived on a branch that was not
the repository's default and HACS only ever tracks the default. That has since
changed - feature/ha-custom-component is the default branch now - so HACS
picks the integration up normally as a custom repository, and releases are
offered as updates to existing installs.

The old note stayed behind after the branch change and was actively steering
people to the manual-install path for no reason. Removed from the integration
README and the repository README; RELEASING.md now records why the default
branch matters, so it does not get moved back by accident.

Documentation only - no functional change.