Update README.md

Added info on why fork is necessary.

Author: BeeDice

README.md

@@ -1,3 +1,6 @@
+# FORK INFO
+The check for `window.__coverage__` only works if `cy.visit()`-ing between every test, since it is done in the `window:load` event in a `beforeEach()` hook. If a test spec does a single `cy.visit()` in a `before()` hook, the load event handler is not hit; if the `cy.visit()` happens in the first test only, coverage is collected for that test, but not for any others.
+
 # @cypress/code-coverage [![renovate-app badge][renovate-badge]][renovate-app] [![CircleCI](https://circleci.com/gh/cypress-io/code-coverage.svg?style=svg)](https://circleci.com/gh/cypress-io/code-coverage)
 
 > Saves the code coverage collected during Cypress tests