Update docs (copied from dockerhub)

Author: Rick Frankel

README.md

@@ -1,3 +1,27 @@
-# alpine-ruby
+# `alpine-ruby`: Minimal Ruby image
 
-This is a simple Dockerfile which will create a base ruby image build on [Alpine Linux](https://www.alpinelinux.org) version 3.2, and its packaged ruby (currently v2.2.3).
+This is a *VERY* small mri ruby 2.2.3 image. It uses the [Alpine Linux](https://www.alpinelinux.org) ruby packages, and has `bundler` and minimal ruby packages installed.
+
+## Using this package
+ 
+``` Dockerfile
+FROM cybercode/alpine-ruby:2.2
+CMD["/mycommand"]
+```
+
+Unlike the [Official Ruby Image](https://hub.docker.com/_/ruby/) or [tinycore-ruby](https://hub.docker.com/r/tatsushid/tinycore-ruby/), it does not create any users or do `ONBUILD` magic and the `CMD` defaults to `irb`.
+
+### Using C-based gems
+
+This image does not contain a compiler, etc. The best way to install C-based gems is to install the compiler chain and any development libraries required, run bundle install and remove the libraries all in one `RUN` command. That way the the final image will stay small.
+
+For example,  if you are using the `pg` and `nokogiri` gems:
+
+``` Dockerfile
+BUILD_DEPS="build-dependencies build-base ruby-dev libc-dev linux-headers \
+    openssl-dev postgresql-dev libxml2-dev libxslt-dev"
+RUN apk --update add --virtual  $BUILD_DEPS && \
+    sudo -iu app bundle config build.nokogiri --use-system-libraries && \
+    sudo -iu app bundle install --path vendor/bundle && \
+    apk del $BUILD_DEPS && unset BUILD_DEPS
+```