#433 Inform Linux users about Java version needed

[Anton]

Taskunifier doesn't seem to work when using the Open Java SDK coming with Ubuntu.
The GUI becomes soon unusuable. To the user this appears to be like very badly
implemented software (although the software is once this is corrected very good software), and it is likely to result in very negative ratings (e.g. in
the toodledo rating of software).

I recommend adding to the readme file information that one should install the Oracle Java JDK, and giving information which java version is to be used.
How to switch depends on the Linux distribution, for Ubuntu information can be found here: https://help.ubuntu.com/community/Java