README: Use go get -d to retrieve v8worker2

[dlespiau]

The very first time one tries to do a go get, it fails with:

  # pkg-config --cflags  -- v8.pc
  Failed to open 'v8.pc': No such file or directory
  No package 'v8.pc' found
  pkg-config: exit status 1

because go get will try to build the package and v8 hasn't been built yet and
the cgo pass of the compilation can't find the pkg-config file.

To avoid that, use the -d option: go get will only download the package and not
build it.