Update readme to include steps for how to contribute

[nitrohorse]

Having clear steps for new users for what exactly they need to do to add a new post would be helpful I think. I'm assuming they need to create new Markdown post and author files in https://github.com/privacytoolsIO/blog.privacytools.io/tree/master/collections?

[jonaharagon]

These steps should probably on an actual Jekyll page on the website rather than the readme. Maybe even on the About page in #1.