Clarify contributing guidelines

[JordanMartinez]

The following need to be added/clarified in CONTRIBUTING.md:

Issue Reporting

• Environment info (OS, RTFX version, Java version)
• If issue is a bug: at the very least, provide code as an Application that reproduces bug/issue; at the very best, provide code as a TestFX test that reproduces the bug/issue
• If issue is a question, explain the desired outcome and a photo showing that (if applicable)

PR guidelines

• style guidelines (usage of blank lines in code [e.g. before/after curly braces] among other things)

PR guidelines for demo package

• Add the suffix "Demo" to the demo, so that people know which class is the demo and which classes are things used eventually in the demo
• Modulate the code to multiple classes (if things cannot be set up in Application#start, helper methods, or code would be better understood as a separate class.
• If demo is a single class, put it in topmost package; if it requires multiple classes, put it and the classes it uses into its own package.

[JordanMartinez]

PR guidelines
style guidelines (usage of blank lines in code [e.g. before/after curly braces] among other things)

This guideline is better enforced (like what TestFX does) than explained

[JordanMartinez]

Since the style guideline is it's own separate issue, I'll open a new one for that.