Descent 3 Build

Copy command-line documentation into `USAGE.md` & add documentation for `-additionaldir` #1898

Annotations

10 warnings

'<': signed/unsigned mismatch

'initializing': conversion from 'size_t' to 'int', possible loss of data

'return': conversion from 'size_t' to 'int', possible loss of data

'type cast': pointer truncation from 'void *' to 'uint32_t'

'type cast': truncation from 'void *' to 'uint32_t'

'e': unreferenced local variable

'e': unreferenced local variable

'e': unreferenced local variable

'argument': conversion from 'int64_t' to 'unsigned int', possible loss of data

'argument': conversion from 'size_t' to 'DWORD', possible loss of data