Add basic build instructions

[aggregate1166877]

Would it be possible to add basic build instructions to the readme? Doesn't need to be anything too in-depth; just a one-liner for the needed build command.

I assume this is built with either make or cmake, but neither work for me:

$ make
make: *** No targets specified and no makefile found.  Stop.

$ cmake
Usage

  cmake [options] <path-to-source>
  cmake [options] <path-to-existing-build>

[aggregate1166877]

Seems I've figured it out, at least Linux Mint:

# Install dependencies:
apt install build-essential qt5-default make cmake

# Prep and build:
cmake .
make

# Run:
./xcursor-viewer

Works very nicely :)