mruby-compiler: add README.md │

Co-authored-by: Gemini <gemini@google.com>

mrbgems/mruby-compiler/README.md

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+# mruby-compiler
+
+This mrbgem provides the mruby compiler, which is responsible for parsing Ruby
+code and generating mruby bytecode.
+
+## Functionality
+
+The `mruby-compiler` gem includes the following components:
+
+- **Parser:** Translates Ruby source code into an abstract syntax tree (AST).
+- **Code Generator:** Traverses the AST to produce executable mruby bytecode.
+- **`mrbc` executable:** A command-line tool for compiling `.rb` files into
+  `.mrb` bytecode files.
+
+## Usage
+
+The `mrbc` (mruby-bin-mrbc) executable will generate compiled binary from Ruby
+programs via this gem.
+
+Example of using `mrbc`:
+
+```sh
+# Compile a Ruby script to bytecode
+bin/mrbc my_script.rb
+
+# Run the compiled script
+bin/mruby my_script.mrb
+```