Documentation for systemverilog plugin

[alaindargelas]

1. The -defer option should be documented explicitly as:
   All files compiled in an invocation of systemverilog -defer file1.v file2.v are compiled as a single compilation unit.
   Several compilation units can be created with separate invocations of "systemverilog -defer ..."

I would have called the option -unit instead of -defer, Compilation units are understood by the Verilog community.

2. Can you give examples on how one pass include paths (-I, +incdir) , library paths (-y), library file extension (+libext+) to the read_systemverilog command? The documentation only talks about passing systemverilog files.