Document secure cookie usage and restructuring readme

[rekire]

As requested in #52 I updated the README:

• Add the hint how to set secure cookies
• Reformat code where the language was incorrectly set
• Change structure for a better reading flow
• Using level 3 headlines for a better way to link to the documentation

You are actually using a different CookieManager implementation, I fixed the URL for you too.
I hope you are happy with my changes, it became a bit more then expected in first place.

Resolves #52

[fryette]

@rekire Thank you very much, applied and released new version!