Show how to build riscv-tools in parallel (#530)

Make it easier to build riscv-tools faster on multicore machines

README.md

@@ -37,6 +37,7 @@ of riscv-tools:
     $ cd rocket-chip/riscv-tools
     $ git submodule update --init --recursive
     $ export RISCV=/path/to/install/riscv/toolchain
+    $ export MAKEFLAGS="$MAKEFLAGS -jN" # Assuming you have N cores on your host system
     $ ./build.sh
     $ ./build-rv32ima.sh (if you are using RV32).