Add make targets for serving docs locally

docs/Makefile

@@ -0,0 +1,18 @@
+.PHONY: ruby
+ruby:
+	@ if [ ! $$(which ruby) ]; then \
+			echo "Please install ruby version 2.1.0 or higher"; \
+		fi
+
+.PHONY: bundle
+bundle: ruby
+	@ if [ ! $$(which bundle) ]; then \
+			echo "Please install bundle: `gem install bundler`"; \
+		fi
+
+vendor/bundle: bundle
+	bundle install --path vendor/bundle
+
+.PHONY: serve
+serve: vendor/bundle
+	bundle exec jekyll serve

docs/README.md

@@ -0,0 +1,13 @@
+# Docs
+
+This folder contains our Jekyll based docs site which is hosted at
+https://pusher.github.io/oauth2_proxy.
+
+When making changes to this docs site, please test your changes locally:
+
+```bash
+make serve
+```
+
+To run the docs site locally you will need Ruby at version 2.1.0 or
+higher and `bundle` (`gem install bundler` if you already have Ruby).