Add setup info to readme #2

if you clone the repo (without initialising its submodule) and try to configure it you hit the following cmake error

CMake Error at CMakeLists.txt:42 (message):
  Failed to find ruby for RUBY_VERSION=3.2

We could suggest that this is likely caused buy not having initialised the submodule. A git command could be suggested

We should also update the readme so this doesn't happen in the first place. We should explain that the repo can be configured and built in order to contribute to it. Or if it is to be used in other projects, people can add the relevant folder to the include paths and they should be good to go.