mruby-toplevel-ext: add README.md

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mrbgems/mruby-toplevel-ext/README.md

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+# mruby-toplevel-ext
+
+mruby-toplevel-ext is a gem that modifies the toplevel in mruby by making the `include` method private. This means modules must be included directly into `Object` (e.g., `Object.include MyModule`) rather than at the toplevel.
+
+## Usage
+
+After installing this gem, the `include` method at the toplevel (main) becomes private. This is the primary change provided by this gem.
+
+Consider the following example:
+
+```ruby
+module MyModule
+  def my_method
+    puts "Hello from MyModule!"
+  end
+end
+
+# This will result in a NoMethodError because include is private:
+# include MyModule
+# my_method
+
+# Instead, you must include the module directly into Object:
+Object.include MyModule
+my_method
+# => Hello from MyModule!
+```
+
+This encourages a more explicit way of extending the global object space.
+
+## License
+
+The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).