Add common errors section to User Guide.

Also clarify that 64-bit JVMs are preferred.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>
Merge branch 'feedback' of github.com:mheon/checksims into feedback
Add paragraph header for JVM arguments.

Clarify that we're not explaining all of them.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>