Build instructions unclear

[Viterkim]

When trying to build, the readme says to do the following:

mkdir mybuild       # Create a directory to hold the build output.
cd mybuild
cmake ${OPEN_VCDIFF_DIR}  # Generate native build scripts.

This is however not correct, and the correct steps on linux are:

mkdir mybuild
git submodule update --init
cd mybuild
cmake ..
make

If the gitsubmodule update --init command isn't run it will complain about missing files.

[Viterkim]

nvm i can't read it's at the top of the readme