add tests/troubleshooting.md guide on how to get E2E tests to run

Author: janosh

tests/troubleshooting.md

@@ -0,0 +1,24 @@
+# Troubleshooting
+
+## Webdriver
+
+If you're seeing errors like
+
+```sh
+ERROR    dash.testing.browser:browser.py:433 <<<Webdriver not initialized correctly>>>
+```
+
+or
+
+```sh
+E   selenium.common.exceptions.WebDriverException: Message: unknown error: cannot find Chrome binary
+```
+
+try manually installing `chromedriver` and/or `google-chrome`. On macOS, this can be done with `brew`:
+
+```sh
+brew install --cask chromedriver
+brew install --cask google-chrome
+```
+
+macOs may complain that it cannot verify the security of these executables in which case you can manually override and give permission in 'System Preferences > Security & Privacy'.