Add updating `git submodule` on first run to instructions

[sudhanshu2]

Building without initializing git submodule leads to the following error

The source directory
    /spatter/external/argtable3
does not contain a CMakeLists.txt file.

This error is fixed by running the following commands in the project directory

git submodule init
git submodule update

It would be nice if running these commands were a part of the readme.

[jyoung3131]

Hi, Sudanshu,

Can you please add this info to the README or the testing wiki page and do a PR for it?

[jyoung3131]

Addressed with PR #68.