Add doc for creating new adapters

README.markdown

@@ -351,6 +351,145 @@ to one of the values specified in the url whitelist like so:
 DatabaseCleaner.url_whitelist = ['postgres://postgres@localhost', 'postgres://foo@bar']
 ```
 
+## How to add a new adapter
+
+_Note the following tutorial is for database_cleaner version 2 and above_
+
+Every adapter is a separate gem. So a creation of an adapter will follow the general rules of a gem making
+
+### Naming
+A convention for naming a gem extesions is with [dash](use-dashes-for-extensions).
+For example, `database_cleaner-redis` or `database_cleaner-neo4j`
+
+### Bootstrapping a gem
+We will use `bundle` to bootstrap a gem. This will produce all the initial files needed for a gem creation
+
+```
+bundle gem database_cleaner-adapter
+```
+#### Modifying .gempspec
+You need to add a couple of dependecies to `.gempspec`:
+* `database_cleaner-core`
+* Adapter you're creating this gem for
+
+```
+  spec.add_dependency "database_cleaner-core", "2.0.0"
+  spec.add_dependency "adapter", "some version if required"
+```
+
+#### 
+
+### File structure
+
+Inside `lib/database_cleaner/adapter`. You will need to create a few files
+
+* `base.rb`
+* Separate files for each strategy you have
+
+The file structure you end up with look something like this
+
+```
+\-lib
+  \-database_cleaner
+    \- adapter
+      \- base.rb
+      \- truncation.rb
+      \- deletion.rb
+      \- transaction.rb
+      \- version.rb
+    \- adapter.rb
+```
+
+#### base.rb
+
+File `base.rb` **must** have following methods
+  * class methods `.default_strategy` and `.available_strategies`
+  * instance methods `#db=`, `#db`, `#cleaning`
+
+`DatabaseCleaner::Generic::Base` will add some of those don't forget to include it.
+
+So, in the end you may end up with the class that will look something like this
+
+```ruby
+require 'database_cleaner/generic/base'
+module DatabaseCleaner
+  module Adapter
+    def self.available_strategies
+      %w[transaction truncation deletion]
+    end
+
+    def self.default_strategy
+      :truncation
+    end
+
+    module Base
+      include ::DatabaseCleaner::Generic::Base
+
+      def db=(desired_db)
+        @db = desired_db
+      end
+
+      def db
+        @db || :default
+      end
+    end
+  end
+end
+```
+
+### Strategy classes
+
+Each strategy **must** have the following instance methods
+  *  `#clean` -- where the cleaning happens
+  *  `#start` -- added for compatability reasons, do nothing if you don't need to. This method might be included with `::DatabaseCleaner::Generic::Truncation` or `::DatabaseCleaner::Generic::Transaction`
+
+Given that we a creating a strategy for truncation. You may end up with the following class
+
+```ruby
+module DatabaseCleaner
+  module Adatper
+    class Truncation
+      include ::DatabaseCleaner::Adatper::Base
+      include ::DatabaseCleaner::Generic::Truncation
+
+      def clean
+      end
+    end
+  end
+end
+```
+
+Thats about it for the code needed to have your own adapter
+
+### Testing
+
+To make sure that custom adapter adheres to the Database Cleaner API database_cleaner provides an rspec shared example.
+
+You need to create a file `database_cleaner/adapter/base.rb`
+
+With the following content
+
+```ruby
+require 'database_cleaner/adapter'
+require 'database_cleaner/spec'
+
+module DatabaseCleaner
+  RSpec.describe Adapter do
+    it_should_behave_like "a database_cleaner strategy"
+  end
+end
+```
+
+### What's next
+Now you should be well set up to create you own database_cleaner adatper.
+Also don't forget to take a look at the already created adapters if you encounter any problems.
+
+When you are done with the your adapter, only a few things left to do
+  * Create a repository with your code
+  * Push code to rubygems
+  * Add your adapter to the [list](https://github.com/DatabaseCleaner/database_cleaner#list-of-adapters)
+
+
 ## COPYRIGHT
 
 See [LICENSE](LICENSE) for details.