README: Specify that PyOCR only works on GNU/Linux systems

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
README: Specify that PyOCR only works on GNU/Linux systems & alike.

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Merge branch 'master' of github.com:jflesch/pyocr