Add instruction to run Percy on forked repositories

[luixxiul]

Your use case

What would you like to do?

I would like to have instruction to run Percy on forked repositories, including about what to install, how to set it up, etc.

Why would you like to do it?

I wanted to understand how to set up Percy (with my account there) for my own forked repository so that I could test the change there first to ensure that it should not introduce a flaky test to the main repository.

I tried once without whether knowing running it on a local repository would be even possible, but without an introduction I could not proceed more than linking my GitHub repository on my Percy preference (local PR against develop branch just skipped Percy test).

How would you like to achieve it?

Have you considered any alternatives?

No response

Additional context

No response

[Johennes]

Closing this as not planned for the moment. I don't believe we want to support running it on forks and we're reconsidering the value of Percy in general with https://github.com/vector-im/wat-internal/issues/56.