update README.md to mention submodules and contribution notes.
update README.md to mention submodules and contribution notes.

FFIG/ffig,jbcoe/C_API_generation,jbcoe/C_API_generation,FFIG/ffig,FFIG/ffig,jbcoe/C_API_generation,jbcoe/C_API_generation,FFIG/ffig,FFIG/ffig,FFIG/ffig,FFIG/ffig,FFIG/ffig