Add "git submodule update --init --recursive" in README

[pierricgimmig]

In the build section, it could be useful to add the git submodule update --init --recursive command before cmake -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo. Also, maybe list system dependencies, I had to install libcurl4-openssl-dev for example.

[pierricgimmig]

Also, the README states that "Building the Crashpad Backend requires a C++14 compatible compiler.", it seems it actually requires a C++ 17 compiler.

if(NOT CMAKE_CXX_STANDARD)
	set(CMAKE_CXX_STANDARD 17)
endif()