add usage guidelines for waku content topics

[chaitanyaprem]

Included a small section on usage guidelines for waku content topics.

It was observed that in status communities these guidelines were not followed which is being fixed based on discussion post.
Hence adding an explicit section will help app developers design content-topics accordingly.

[chaitanyaprem]

@jimstir no clue why linter is showing errors even though the files that have been changed in this PR have none. Maybe we can fix the CI action to only show errors on files that have been touched (added/updated) in the PR.

[jimstir]

@jimstir no clue why linter is showing errors even though the files that have been changed in this PR have none. Maybe we can fix the CI action to only show errors on files that have been touched (added/updated) in the PR.

We thought about this but have decided that all markdown files in this repo should follow markdown-linting rules, so any errors should be addressed. Do not worry about the linter, the errors displaying here are being addressed here.