Add Code Style Guidelines to our Contribution Guide

[teobais]

As mentioned in #155 (this particular section) our code styling has no specific format at the moment.

Let's go for the standard one (IntelliJ formatting) and add it to our Contribution Guidelines so that we ensure consistency in the future.

I can work on this as well, but as we know, I'm not able to assign it to myself :-)

[keilw]

Can IntelliJ formatting also be automated? If not, then please let's stick to the Eclipse one we already got here https://github.com/unitsofmeasurement/unit-api/blob/master/src/main/config/eclipse-formatter-config.xml. If something is missing, please adjust.

[teobais]

Good point, I'll have a look at it.

[keilw]

Ok this seems about the Wiki but there also should be something in the Guide Book: unitsofmeasurement/uom-guide#3

[teobais]

It looks like IntelliJ does not provide automated formatting the way Eclipse does. Thus, the only way for Teams which are using more than 1 IDEs is to import the already existing settings file that we have.

Hence, closing this one, but it would be good to mention this required setting at unitsofmeasurement/uom-guide#3 .