Add README note about building Release versions

Updated the README with instructions on how to make a Release build, and a note about how important this is for Windows users who wish to distribute gdosc with their projects.

Addresses issue #12.

Note that users can install MSVC redistributables by following these instructions:

https://learn.microsoft.com/en-AU/cpp/windows/latest-supported-vc-redist?view=msvc-170

However, those will not include the Debug versions of libraries required by Debug-built libgdosc.dll, such as vcruntime140d.dll or msvcp140d.dll (note the 'd' at the end of the names). Building in Release solves the issue. The problem is often not seen by developers (like myself), because we normally have Visual Studio installed along with all the debug (and release) tools and libraries.