[qa] Add readme for end-to-end testing

cypress/readme.md

@@ -0,0 +1,29 @@
+# End-to-end testing Studio
+
+Before you can run the tests you need to add a sesstion token
+so the tests are able to log in.
+
+Open the test studio in Chrome ou
+would normally). Open the "Developer Console", navigate to
+
+1. Open test studio in a browser (by running `npm start` as you
+   normally would)
+2. Open "Developer console" and find the value of the `sanitySession`
+   cookie
+3. Set the environment variable by running
+   ```shell
+   export CYPRESS_SANITY_SESSION_TOKEN='<...>'
+   ```
+   where you replace `<...>` with the value from step 2
+
+Run the tests like so:
+
+```
+$ npm run test-e2e
+```
+
+Or open cypress for interactivity:
+
+```
+$ cypress open
+```