add README with build instrucions

README.md

+# Chaotikum Website
+
+Wiki: https://wiki.chaotikum.org/projekte:chaotikumwebsite
+
+## Build
+
+Install depencies:
+
+	gem install bundler
+	bundle install
+
+Build:
+
+	bundle exec jekyll build
+
+Or launch a webserver with consecutive builds:
+
+	bundle exec jekyll serve