Add installation instructions

Fixes #1

docs/installation.md

@@ -0,0 +1,17 @@
+# Installation
+
+1. Run `vagrant up` or `vagrant up --provision` if needed, to start Vagrant and provision the virtual machine using the `ansible/provision.yml` playbook. This will set up a LAMP stack with Apache, MySQL, PHP and Composer.
+
+    If you try loading the site at this point you will see a `The requested URL was not found on this server` message as the site has not yet been deployed.
+
+    ![A browser showing a 'Not Found' error](images/before-deploy.png)
+
+1. Run the deploy playbook to deploy the application code to the virtual machine. You will be prompted for the Vault password, which is `dransible`.
+
+    ```
+    ansible-playbook ansible/deploy.yml -i ansible/hosts.yml --ask-vault-pass
+    ```
+
+1. Load `http://dransible` in a browser, and you should see a Drupal 8 website.
+
+    ![A browser showing the front page of a Drupal website](images/after-deploy.png)