readme should document Metals setup

[SethTisue]

working on #554 here in the Scala Open Source spree in Paris

I merged #564 to increase the default heap size.. hit that in the first 5 min

• the non-JVM projects may fail (e.g. if node isn't installed), so we should have instructions on how to only work on the JVM build (compat212 is the relevant project)
• we should recommend working in Scala 2.12 to begin with and let CI do the 2.11 compile
• Metals will complain about 2.11 being present, that's normal

[SethTisue]

It wasn't clear to any of us (me, @rjolly, or @AdrienPI) if it was possible to tell Metals to only run the 2.12 JVM tests and ignore the rest. So Metals was fine for editing the code, but we had to switch to sbt and run compat212/test.