edit README

README.md

@@ -22,7 +22,64 @@ Or install it yourself as:
 
 ## Usage
 
-TODO: Write usage instructions here
+### GTIN13
+
+```ruby
+require "jancode"
+
+# Jancode::GTIN13.new("GS1 Company Prefix", "Item Code")
+jancode = Jancode::GTIN13.new("456995111", "617")
+jancode.check_digit # => 9
+jancode.create # => "4569951116179"
+
+# jancode.company_prefix = "456995111"
+jancode.item_code = "618"
+jancode.create # => "4569951116186"
+
+(1..10).each do |item_code|
+  jancode.item_code = "%03d" % item_code
+  puts jancode.create
+end
+# 4569951110016
+# 4569951110023
+# 4569951110030
+# 4569951110047
+# 4569951110054
+# 4569951110061
+# 4569951110078
+# 4569951110085
+# 4569951110092
+# 4569951110108
+```
+
+### GTIN8
+
+```ruby
+require "jancode"
+
+# Jancode::GTIN8.new("GS1 Company Prefix", "Item Code")
+jancode = Jancode::GTIN8.new("491234", "5")
+jancode.check_digit # => 6
+jancode.create # => "49123456"
+
+# jancode.company_prefix = "491234"
+jancode.item_code = "6"
+jancode.create # => "49123463"
+
+(1..9).each do |item_code|
+  jancode.item_code = item_code.to_s
+  puts jancode.create
+end
+# 49123418
+# 49123425
+# 49123432
+# 49123449
+# 49123456
+# 49123463
+# 49123470
+# 49123487
+# 49123494
+```
 
 ## Contributing