Add documentation for PRs from forks

docs/releasing/testing-and-linting.md

@@ -56,4 +56,12 @@ We generate two screenshots for each default example of every component: one wit
 
 The screenshots are public, so they can be checked without logging in. A BrowserStack account is needed to approve or reject any changes. It's the responsibility of the person reviewing the pull request code to approve any visual changes that Percy highlights.
 
-Note: when running the tests locally via `npm test`, Percy commands are ignored and no screenshots are generated. You will see the following message in your command line output: `[percy] Percy is not running, disabling snapshots`.
+When running the tests locally (e.g: via `npm test`), Percy commands are ignored and no screenshots are generated. You will see the following message in your command line output: `[percy] Percy is not running, disabling snapshots`.
+
+### PRs from forks
+When Github Actions is running against a PR from a fork, the Percy secret is not available and therefore no screenshots are generated. Other tests will continue to run as normal. You will see the following messages in the output:
+
+```
+[percy] Skipping visual tests
+[percy] Error: Missing Percy token
+```