Update README to have clear and correct documentation on usage with Windows agents

[izgeri]

At current, the documentation only has one section on using this integration with Windows agents (and it requires using the host/API key flow)

AC:

• The docs are clear about which use cases are supported for Windows agents
• Based on our experience running Puppet with Windows agents using our plugin, any additional helpful information about using our plugin with Windows is also documented
• If any documented flows are not supported in Windows, this is clearly indicated in those sections.

[doodlesbykumbi]

After testing the flows, I think we support the same things for Linux as we do for Windows, which I think the latest README from your branch communicates well.

Based on our experience running Puppet with Windows agents using our plugin, any additional helpful information about using our plugin with Windows is also documented

The only difference is regedit + windows credentials manager (vs /etc/conjur.conf and /etc/conjur.identity for Linux). This is already captured. It might be useful to mention the equivalence which might help users reason about machine identity by analogy.

If any documented flows are not supported in Windows, this is clearly indicated in those sections.

I'm not aware of any flows that are not supported.