mruby-struct: add README.md

The document is written by Google Jules.

mrbgems/mruby-struct/README.md

@@ -0,0 +1,54 @@
+# mruby-struct
+
+This mrbgem provides the `Struct` class, a convenient way to bundle a number of attributes together, much like Ruby's core `Struct` class. It allows you to create simple classes (structs) with a defined set of accessor methods for these attributes.
+
+## Functionality
+
+- Define new Struct classes with a specific set of members.
+- Create instances of these Structs.
+- Access and assign to struct members using accessor methods or by index/symbol.
+- Iterate over members and values.
+- Convert structs to Arrays or Hashes.
+
+## Basic Usage
+
+```ruby
+# Define a new Struct class
+Point = Struct.new(:x, :y)
+
+# Create an instance of the Point struct
+origin = Point.new(0, 0)
+
+# Access attributes
+puts origin.x  # Output: 0
+puts origin[:y] # Output: 0
+
+# Set attributes
+origin.x = 10
+origin[:y] = 20
+
+puts origin.inspect # Output: #<struct Point x=10, y=20>
+
+# Another example
+Customer = Struct.new("Customer", :name, :address)
+joe = Customer.new("Joe Smith", "123 Maple, Anytown NC")
+
+puts joe.name    # Output: Joe Smith
+puts joe.address # Output: 123 Maple, Anytown NC
+```
+
+## Available Methods
+
+Instances of classes created with `Struct.new` have several useful methods, including:
+
+- `members`: Returns an array of symbols representing the names of the instance variables.
+- `each`: Calls a block for each attribute, passing the value.
+- `each_pair`: Calls a block for each attribute, passing the name (symbol) and value.
+- `select`: Returns an array containing values for which the block returns true.
+- `to_a` / `values`: Returns an array containing the values of the struct.
+- `to_h`: Returns a hash mapping member names (symbols) to their values.
+- `length` / `size`: Returns the number of members in the struct.
+- `dig`: Extracts a nested value specified by a sequence of keys.
+- `==`, `eql?`: For comparing struct instances.
+
+For more details on specific methods, please refer to the mruby documentation or the core Ruby `Struct` class documentation, as `mruby-struct` aims for compatibility.