Create json.md

docs/json.md

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+# Serving JSON API
+
+Just use `to_json` method to return.
+
+```ruby
+require "kemal"
+require "json"
+
+# You can easily access the context and set content_type like 'application/json'.
+# Look how easy to build a JSON serving API.
+get "/" do |env|
+  env.content_type = "application/json"
+  {name: "Serdar", age: 27}.to_json
+end
+
+```