add contrib document

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+Hi there,
+
+feel free to contribute.
+
+We have forked the Minimal theme from jekyll and made some changes. Such as the menu bar. For now the menu is hardcoded in html. If you add new sections to the page, you must add the links yourself in `_layouts/default.html`
+
+BTW you can run the site locally.
+
+To run the site you must install ruby 2.2, or higher. In a shell run the following.
+
+```sh
+gem install bundle
+bundle
+bundle exec jekyll serve
+```
+
+This should return a URL that you can visit in your browser.
+
+Alternatively you can use any markdown editor, and it should provide a very close experience to the actual site, sans menu.
+