Disk Image documentation update

[pgodwin]

Firstly, thanks for creating this project. I'm impressed with the compatibility so far, especially with OS/2 working out of the box - this seems to be quite a feet!

Just a note that https://github.com/nepx/halfix/wiki/Disk-Images#modifying-disk-images is out of date

To enable write, set the ALLOW_READWRITE macro in src/drive.c to 1. This will allow changes to all disk images to be written back to the disk.

TODO: make this a run-time option.

Should be:

To enable write, set writeback=1 in your configuration. This will allow changes to all disk images to be written back to the disk.

Thanks again!