Adding 'Advanced JSON Customization' section in the README.md. Creating a runtime type (internal/data/runtime.go) to show how we can customize a JSON field.

internal/data/runtime.go

@@ -0,0 +1,33 @@
+package data
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// Runtime
+/*
+Declare a custom Runtime type, which has the underlying type int32
+(the same as our Movie struct field)
+*/
+type Runtime int32
+
+// MarshalJSON
+/*
+Implement a MarshalJSON() method on the Runtime type so that it satisfies
+the json.Marshaler interface. This should return the JSON-encoded value
+for the movie runtime (in our case, it will return string in the format
+"<runtime> mins").
+*/
+func (r Runtime) MarshalJSON() ([]byte, error) {
+	// Generate a string containing the movie runtime in the required format.
+	jsonValue := fmt.Sprintf("%d mins", r)
+
+	// Use the strconv.Quote() function on the string to wrap it in double
+	// quotes. It needs to be surrounded by double quotes in order to be
+	// a valid *JSON string*.
+	quotedJSONValue := strconv.Quote(jsonValue)
+
+	// Convert the quoted string value to a byte slice and return it.
+	return []byte(quotedJSONValue), nil
+}