Edit contribution guideline to add information about visual tests with Percy

[luixxiul]

Your use case

What would you like to do?

Add policies about visual tests with Percy to contribution guideline (https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md).

• Information about which specific changes require visual tests
• Link to the documentation
• Edit pull request templates such as https://github.com/matrix-org/matrix-react-sdk/blob/develop/.github/PULL_REQUEST_TEMPLATE.md to explain the necessity of visual tests

Why would you like to do it?

Because internal decisions should be published and available openly when it comes to development.

How would you like to achieve it?

Ask someone who are familiar with internal decisions :-D

Have you considered any alternatives?

Until the policies become publicly available, please do not make adding a visual test a requisite for a PR to be accepted.

Additional context

No response

[luixxiul]

the documentation has been improved well enough. closing