Improve docs - disable key file permission check

[starkskalle]

Hey,

I believe it would be great to update the docs with following or similar statement:

By default both key files are checked for correct permissions (chmod 600 or 660
is expected). In case the environment/operating system (e.g. Windows) does not 
support such a setting, the permission check can be disabled:


    // ...
    'private_key' => [
        'key_or_path' => __DIR__ . '/../data/oauth/private.key',
        'key_permissions_check' => false,
    ],
    // ...

This would help developers who are not using linux based OS etc. What do you think?

Thanks

[michalbundyra]

@starkskalle
Good idea. Would you like open PR with suggested change? Thanks!