Move PuTTYGen into the 'What you will need' section

Remove key generation from the PuTTY guide and replace it with a link to a
PuTTYGen tutorial, for koding/docs#89
There are SSH instructions for Windows now!
Merge pull request #93 from ellotheth/issue/89-add-puttygen-to-requirements

Issue 89: Add puttygen to requirements