Update README with basic instructions

Ticket: https://trello.com/c/oN9w1d9g

Author: phansch

README.md

@@ -1,8 +1,8 @@
 # OrderTransformer
 
-Welcome to your new gem! In this directory, you'll find the files you need to be able to package up your Ruby library into a gem. Put your Ruby code in the file `lib/data_transformer`. To experiment with that code, run `bin/console` for an interactive prompt.
+Note: This gem is in maintenance mode and not actively updated.
 
-TODO: Delete this and the text above, and describe your gem
+The order transformer transforms data from an external format to the desired target format.
 
 ## Installation
 
@@ -12,17 +12,64 @@ Add this line to your application's Gemfile:
 gem 'order_transformer'
 ```
 
-And then execute:
+And then run `bundle install`.
 
-    $ bundle install
+## Usage
 
-Or install it yourself as:
+The gem allows you to define data transformations via a DSL like so:
 
-    $ gem install data_transformer
+```ruby
+OrderTransformer::DSL.define :some_shop, "v1.0" do
+  definition do
+    order as: :hash do
+      # The core of the transformation process is implemented by using the `transform` method. This maps the input "OrderNumber" field to the output "order_number" field.
+      transform "OrderNumber", to: "_order_number"
+
+      # By default, values of input data are not required to be set. To
+      # require them, set `optional: false`:
+      transform "OrderNumber", to: "_order_number", optional: false
+
+      # If you need to modify the input data, provide a custom transform instruction:
+      transform "OrderNumber", to: "_order_number", transformer: ->(order_number) { order_number.to_d }
+
+      # The gem also comes with some default transformers that are always included.
+      # See `DefaultTransformers`.
+      # The above can be rewritten as:
+      transform "OrderNumber", to: "_order_number", transformer: to_d
+
+      # Using multiple inputs is also possible:
+      transform "street_1", "street_2", "street_3", to: "_street", transformer: ->(street_1, street_2, street_3) { street_1 || street_2 || street_3 }
+
+    end
+  end
+end
+```
 
-## Usage
+You can collect your own data transformers in a plain Ruby module:
+
+```ruby
+module MyTransformers
+  def presence(chain = nil)
+    chain ||= __create_method_chain(caller(1, 1))
+    chain.add ->(value, *_args) { value.presence }
+  end
+end
+```
+
+and include it in your transformer:
 
-TODO: Write usage instructions here
+
+```ruby
+OrderTransformer::DSL.define :some_shop, "v1.0" do
+  include_transformers MyTransformers
+
+  definition do
+    order as: :hash do
+      transform "OrderNumber", to: "_order_number", transformer: presence
+    end
+  end
+end
+```
 
 ## Development
 
@@ -50,9 +97,3 @@ To install this gem onto your local machine, run `bundle exec rake install`. To
 - create branch from master
 - bump version to next pre `be gem bump -v pre --pretend` and after check `be gem bump -v pre`
 - merge branch (merge commit!) into development
-
-## Contributing
-
-Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/data_transformer.
-
-