documentation should suggest to update submodule

[rngadam]

CMake Error at cmake/SetCV2CGDependencies.cmake:30 (message):
  Please specify a valid root directory for LCH
  (https://github.com/simbaforrest/lch.git)!
Call Stack (most recent call first):
  CMakeLists.txt:27 (include)

documentation should have:

git submodule update --init --recursive

[simbaforrest]

Thanks for the suggestions. Will update the documentation later. I'm traveling a lot recently...