Simplify and document a procedure to run integration tests on real browsers

[php-coder]

[php-coder]

For example, I tried to run the tests in Firefox and the following modifications were required:

• set BROWSER variable to firefox in pom.xml
• download geckodriver binary from https://github.com/mozilla/geckodriver
• pass -Dwebdriver.gecko.driver=$PWD/geckodriver.exe property

After that I came across the issue where sessions weren't persisted on disk and it seems like I should use an older Firefox version: https://stackoverflow.com/questions/57947509/cant-find-profile-directory-error-while-running-some-tests-in-a-firefox-brows

[php-coder]

Perhaps, we can use https://github.com/Ardesco/driver-binary-downloader-maven-plugin to simplify a driver's downloading.

[php-coder]

For Chrome it's similar to firefox:

• chrome
• https://github.com/SeleniumHQ/selenium/wiki/ChromeDriver
• -Dwebdriver.chrome.driver=$PWD/chromedriver.exe