Update readme with submodule checkout instructions for source build
Update readme with submodule checkout instructions for source build

kripken/node-lz4,kripken/node-lz4,kripken/node-lz4,pierrec/node-lz4,pierrec/node-lz4,pierrec/node-lz4,kripken/node-lz4,pierrec/node-lz4