Update readme about event-loop support

[inancgumus]

The current README explains that k6 doesn't have event-loop support.

• We might want to update the relevant sections of the README that we do have, before the next release.
• We also need to update the Browser.on support.
• We also need to update the k6 browser documentation.

For example:

Are all features of Playwright supported? No. Playwright's API is pretty big and some of the functionality only makes sense if they're implemented as async operations: event listening, request interception, waiting for events, etc. As grafana/k6#882 yet, the xk6-browser API is synchronous right now and thus lacks some of the functionality that requires asynchronicity.

[imiric]

I did the minor changes in #295, but for updating the API docs we have grafana/k6-docs#582. So unless there's something else missing from the README, we can close this with #295.