update README

Author: rubysolo

README.md

@@ -11,3 +11,63 @@ If [available in Hex](https://hex.pm/docs/publish), the package can be installed
         def deps do
           [{:excelerator, "~> 0.0.1"}]
         end
+
+## Usage
+
+  1. Import `Excelerator.Workbook` into your module, then generate the XML content:
+
+      workbook do
+        worksheet "My Sheet" do
+          row do
+            cell "Name"
+            cell "Status"
+          end
+
+          row do
+            cell "Elixir"
+            cell 1
+          end
+        end
+      end
+
+      # => <?xml version="1.0" encoding="UTF-8" ?><Workbook ...
+
+## Usage with Phoenix
+
+  1. Use `Excelerator.Phoenix` in your controller:
+
+      defmodule MyApp.MyController do
+        use MyApp.Web, :controller
+        use Excelerator.Phoenix
+
+  1. Import `Excelerator.Workbook` into your view module:
+
+      defmodule MyApp.MyView do
+        use MyApp.Web, :view
+        import Excelerator.Workbook
+
+  1. Alternatively, add the use / import statements to the appropriate sections
+     of `web/web.ex`, which will make excelerator available in all controllers/
+     views.
+
+  1. Call `xls` in your controller action to send content as a file download:
+
+      def show(conn, %{"format" => "xls"} = params) do
+        xls conn, "show.xls", favorite: "Elixir"
+      end
+
+  1. Create `web/templates/RESOURCE/NAME.xls.exs`
+
+      workbook do
+        worksheet "My Sheet" do
+          row do
+            cell "Name"
+            cell "Status"
+          end
+
+          row do
+            cell assigns.favorite
+            cell 1
+          end
+        end
+      end