Add windows install instructions

[ehayes2000]

Specify requirement for use on windows.

[LudovicRousseau]

You do not have to install OpenSC on Windows (or any other OS) to use PyKCS11.
OpenSC is not the only PKCS#11 library.

Or I missed something?

[ehayes2000]

You are right there are other PKCS#11 implementations.

On windows PKCS#11 is not installed by default and PyKCS11 cannot be imported until a PKCS#11 implementation is added to the path. After spending a considerable amount of time yesterday resolving this import issue I amended the README with my fix. There may be a more accurate description of this fix, but regardless it should be made clear that PKCS#11 must be installed with this library.

[LudovicRousseau]

What do you mean by "PyKCS11 cannot be imported"?

import PyKCS11

should always work.

[ehayes2000]

No. import PyKCS11 did not work until I installed OpenSC I got an error stating that some module could not be found. I'm using a different computer to write this so I'll try to reproduce the issue for you tomorrow and provide more details.

It looks like I should have made this an issue instead of a PR, my mistake.