Add hint for users of ImageMagick regarding conversion to bmp

```
$ convert scan.png scan.bmp
$ paperback-cli --decode -i scan.bmp -o /tmp/myfile
Decoding scan.bmp into /tmp/myfile
Reading scan.bmp...
Unsupported bitmap type: scan.bmp
```
it would be useful to read in `README` that BMP3 format is the one that is supported:
```
$ convert scan.png BMP3:scan.bmp
```