Document migration path from robotframework-selenium2library-java

[php-coder]

Hi, thank you for maintaining this library!

I'm migrating away from https://github.com/MarkusBernhardt/robotframework-selenium2library-java and I found that I have to change Library name from Selenium2Library to SeleniumLibrary.

Could this be documented in the Differences section? IMHO it could be useful for newcomers.

Thanks!