message: Add submodules description
diff: +## Cloning a Project with Submodules
  +
  +Please, note - [git submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules) are used in this project.
  +
  +When you clone such a project, by default you get the directories that contain submodules, but none of the files within them.
  +So, in order to build the project, you need either:
  + * clone repo with all submodules altogether (use `--recursive`)
  +```bash
  +git clone --recursive https://github.com/yandex/clickhouse-odbc
  +```
  + * or add submodules manually after main project cloned - in the root of source tree run:
  +```bash
  +git submodule init && git submodule update
  +```
  +
  ## Building (Linux)
  1. Install unixodbc >= 2.3.0
  