Add mention of SeleniumManager to README

[pgagne]

Selenium has a built in webdriver/browser management now in the form of Selenium Manager. The README Browser drivers section however suggests using WebdriverManager which seems to not have been updated recently.

As someone who used to do a lot of stuff with selenium and is coming back to it, this was something that caused me to research WebdriverManager instead of SeleniumManager, but perhaps it's more widely known. But I think an update might be in order.