Add guidance for SDK README usage examples content

[owenpearson]

Update sdk-readme.md to be more prescriptive about which features to include in the 'Usage' section

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2899

[QuintinWillison]

Triggered by this Slack thread (internal).