Document and specify #macro and other pre-processor directives

[siraben]

After reading parts of the source, I managed to piece together a rough understanding of the pre-processor. I have not been able to find documentation on it, however. The README should include an explanation of #macro, or link to appropriate documentation.

[SimonEast]

I was looking for a similar thing. In case anyone else comes across this thread, I ended up using Brass, a similar assembler to spasm-ng. Brass has much better documentation, which covers the assembler directives. Many are similar (but not identical) to spasm-ng. I think Brass might even have a lot more.