Updated OpenJFX buld instructions

[Kevin Rushforth]

I did a first pass over the build instructions on the OpenJFX Wiki [1]. 
I think I cleaned up the worst of the inaccuracies, and added some 
information that will make it easier to build.

I'm not the best person to see whether anything important is missing, 
though. Someone less familiar with the build should look it over and let 
me know what I've missed. I won't be able to get back to this until 
after the holidays and after JDK 10 RDP1 is over, but will take another 
pass at it then and incorporate the feedback.

-- Kevin

[1] https://wiki.openjdk.java.net/display/OpenJFX/Building+OpenJFX