Clarify how to notify on pull request events

[dbarnett]

Can you provide an example of notifying Discord on pull request? Is that supposed to be a supported event type?

It used to work for me using the pull_request event type, but related to a GitHub security change that broke recently with an error Env var DISCORD_WEBHOOK not defined, apparently unable to access secrets (see #71). I tried changing it to pull_request_target, and that now runs without errors but doesn't post anything to Discord.