add : Dokku deployment environment + instructions

This implements small and basic changes to deploy frab with Dokku.

Gemfile

@@ -1,4 +1,5 @@
 source 'https://rubygems.org'
+ruby '2.2.3'
 
 gem 'rails', '~> 4.2'
 

Procfile

@@ -0,0 +1 @@
+web: bundle exec rails server -p $PORT -e $RACK_ENV

README.dokku.md

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+# Deploying Frab with Dokku
+
+Dokku is a Platform-as-a-Service (PaaS) engine which allows for simple `git push` deployments.
+It builds on Heroku's `buildpack`s and is higly customizable.
+
+To deploy a Frab application with `dokku`, please proceed as follows:
+
+## 1. Setting up Dokku
+
+Given you have access to your Dokku service via a simple alias (`alias dokku='ssh -t dokku@<DOKKU_HOST>'`) and `dokku version` works, do the following:
+
+```
+dokku create <APP_NAME>
+```
+
+The `.env` environment would then look similarily to
+
+```
+SECRET_KEY_BASE=<should_be_longer_than_32_chars, 'pwgen 32' suffices>
+FRAB_HOST=<APP_FQDN=APP_NAME.DOKKU_HOST>
+FRAB_PROTOCOL=<http|https>
+FROM_EMAIL=<sender@host.tld>
+SMTP_ADDRESS=<SMTP MX>
+SMTP_PORT=25
+SMTP_NOTLS=<true|false>
+SMTP_USER_NAME=
+SMTP_PASSWORD=
+BUNDLE_WITHOUT=development:test:mysql:sqlite3
+RAILS_SERVE_STATIC_FILES=true
+```
+
+Pipe this configuration to dokku with
+
+    dokku config:set frab `paste -d " " -s .env`
+
+Make sure you are using a branch which provides Ruby 2.2.3 in the `Gemfile` and the `web` process (only) in the `Procfile`.
+
+```
+dokku postgresql:create <DB_NAME>
+dokku postgresql:link <APP_NAME> <DB_NAME>
+```
+
+## 2. Setting up frab
+
+Then use the output of the second command to set up your `production` database connection in `config/database.yml`:
+
+```
+production:
+  adapter: postgresql
+  encoding: unicode
+  database: <DB_NAME>
+  username: <DB_USER>
+  password: <DB_PASS>
+  host: postgresql
+  pool: 5
+  timeout: 5000
+```
+
+and force commit this file to your git tree, until the use of a `DATABASE_URL` environment variable is implemented.
+
+## 3. Deploying Frab
+
+    git remote add dokku dokku@<DOKKU_HOST>:<APP_NAME>
+    git push dokku master
+
+After this has completed successfully, you need to manually load the database schema with
+
+    dokku run frab bundle exec rake db:setup
+
+---
+
+That's it. Your application should be running at `<PROTO>://<APP_NAME>.<DOKKU_HOST>`.