Documentation is wrong

Both readme.md and the help text when running wadext.exe declare that the proper syntax of using wadext is as follows:

`wadext [options] filename`

However, attempting to run `wadext -nogfxconvert -nosndconvert filename.wad` prints the following error:

```-nogfxconvert: unable to open```

The _proper_ syntax to use wadext, counter to all documentation, should be `wadext filename [options]`.