mruby-bin-mirb: add README with tab completion documentation

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

mrbgems/mruby-bin-mirb/README.md

@@ -0,0 +1,94 @@
+# mruby-bin-mirb
+
+mirb (mruby interactive) is an interactive Ruby shell for mruby.
+
+## Usage
+
+```
+mirb [options]
+```
+
+### Options
+
+- `-v` - print version and exit
+- `-d` - set `$DEBUG` to true
+- `-r library` - load the library before executing
+- `--verbose` - verbose mode
+
+## Tab Completion
+
+mirb supports context-aware tab completion when built with a readline library.
+
+### Supported Completions
+
+- **Methods on objects**: Type an expression followed by `.` and press Tab
+
+  ```
+  > "hello".up<Tab>
+  upcase  upcase!  upto
+  ```
+
+- **Local variables**: Variables defined in the session
+
+  ```
+  > my_var = 123
+  > my<Tab>
+  my_var
+  ```
+
+- **Global variables**: Press Tab after `$`
+
+  ```
+  > $std<Tab>
+  $stdout  $stderr  $stdin
+  ```
+
+- **Constants and classes**: Capital letter followed by Tab
+
+  ```
+  > Str<Tab>
+  String  Struct
+  ```
+
+- **Ruby keywords**: At the start of expressions
+
+  ```
+  > cla<Tab>
+  class
+  ```
+
+### Readline Library Support
+
+Tab completion works with:
+
+- **GNU readline** (default on Linux)
+- **libedit** (default on macOS/BSD)
+- **linenoise** (lightweight alternative)
+
+### Configuration
+
+The readline library can be configured via the `MRUBY_MIRB_READLINE` environment variable:
+
+```bash
+# Auto-detect (default)
+rake
+
+# Force specific library
+MRUBY_MIRB_READLINE=readline rake    # GNU readline
+MRUBY_MIRB_READLINE=libedit rake     # libedit
+MRUBY_MIRB_READLINE=linenoise rake   # linenoise
+
+# Disable readline (plain input mode)
+MRUBY_MIRB_READLINE=none rake
+```
+
+### Notes
+
+- Completion evaluates receiver expressions to determine available methods
+- Only simple receivers (variable names, constants) are evaluated for safety
+- Complex expressions like `obj.method().` are not completed to avoid side effects
+- File path completion in `require`/`load` statements is planned for future versions
+
+## License
+
+MIT License - see the mruby LICENSE file.