Add "java" as explicit dependency

[Pomax]

Not everyone has java installed (for reasons ranging from literally never needing it, to not wanting the security holes it opens up, to some other reason), so mentioning that there is a hard dependency on java in the README.md and installation instructions on the github.io page is an excellent idea.

[sipayRT]

hi

java is selenium-standalone deps, not gemini. You can use SauceLabs or BrowserStack without java

will add it in docs, tnx

[Pomax]

I know, but it's still a dependency worth pointing out because a lot of people expect instructions like these to cover everything they need to do in order to go from "I have nothing installed" to "I have a working copy of [thing]".

Thanks for adding it to the docs!