mruby-env: add README.md

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-env/README.md

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+# mruby-env
+
+ENV object for environment variable access.
+
+This gem is a built-in replacement for [iij/mruby-env](https://github.com/iij/mruby-env),
+providing a superset of its API.
+
+## Methods
+
+`[]`, `[]=`, `assoc`, `clear`, `delete`, `each`, `each_key`,
+`each_value`, `empty?`, `fetch`, `filter`/`select`, `freeze`,
+`has_key?`/`include?`/`key?`/`member?`, `has_value?`/`value?`,
+`inspect`, `key`, `keys`, `length`/`size`, `merge!`/`update`,
+`rassoc`, `reject`, `replace`, `slice`, `store`, `to_a`, `to_h`,
+`to_s`, `values`
+
+ENV includes `Enumerable` via `mruby-enumerator`.
+
+## Example
+
+```ruby
+ENV["MY_VAR"] = "hello"
+ENV["MY_VAR"]               #=> "hello"
+ENV.delete("MY_VAR")        #=> "hello"
+
+ENV.keys                    #=> ["PATH", "HOME", ...]
+ENV.each { |k, v| puts "#{k}=#{v}" }
+ENV.select { |k, v| k.start_with?("RUBY") }
+```
+
+## License
+
+MIT License - see the mruby license.