Update readme about mruby version and file structure.

Author: gemmaro

README.md

@@ -2,8 +2,9 @@
 
 This is a Ruby library for the GNU gettext PO files.  This is a C
 binding of the libgettextpo library, which is provided by the GNU
-gettext package.  It is designed to promote Ruby's idiomatic coding
-style while avoiding memory leaks.
+gettext package.  The API of this gem is designed to promote Ruby's
+idiomatic coding style while avoiding memory safety.  Currently this
+supports both CRuby and mruby.
 
 ## Installation
 
@@ -23,17 +24,22 @@ apt search libgettextpo
 #   process PO files - shared library
 ```
 
-Install the gem and add to the application's `Gemfile` by executing:
+For CRuby:
 
-```bash
-bundle add gettextpo
-```
+* Install the gem and add to the application's `Gemfile` by executing:
 
-If Bundler is not being used to manage dependencies, install the gem by executing:
+  ```bash
+  bundle add gettextpo
+  ```
 
-```bash
-gem install gettextpo
-```
+* If Bundler is not being used to manage dependencies, install the gem by executing:
+
+  ```bash
+  gem install gettextpo
+  ```
+
+For mruby, modify `build_config.rb` as you like, set `MRUBY_SRC` to
+mruby source repository, and run `./bin/compile` or `./bin/test`.
 
 ## Usage
 
@@ -56,14 +62,12 @@ end
 ```
 
 Please refer to the [API documentation][api] and test cases for
-details.
+details.  The mruby version has same API.
 
 [api]: https://gemmaro.github.io/ruby-gettextpo/
 
 ## Development
 
-`./bin/debug` to debug when segmentation fault.
-
 After checking out the repo, run `bin/setup` to install
 dependencies. Then, run `rake test` to run the tests. You can also run
 `bin/console` for an interactive prompt that will allow you to
@@ -75,6 +79,42 @@ install`.  To release a new version, update the version number in
 create a git tag for the version, push git commits and the created
 tag, and push the `.gem` file to [rubygems.org](https://rubygems.org).
 
+File structure:
+
+```text
+  bin               : script ifles
+  build_config.rb   : for mruby
+* build_config.rb.lock
+  CHANGELOG.md
+  COPYING
+* doc               : generated by RDoc from CRuby sources
+  ext               : for CRuby
+  Gemfile           : for CRuby
+  gettextpo.gemspec : for CRuby
+  include           : for mruby
+  lib               : for CRuby
+  mrbgem.rake       : for mruby
+  mrblib            : for mruby
+* pkg               : generated by RubyGem
+  Rakefile          : for CRuby
+  README.md
+- sample.rb         : for debug purpose especially with GDB
+  sig               : for CRuby
+  src               : for mruby
+  test              : common test cases for both CRuby and mruby
+  test.cruby        : CRuby specific tests
+* tmp
+- .env
+  .env.example
+  .envrc            : Direnv config
+  .clang-format
+  .dir-locals.el    : Emacs config
+  .gitignore
+
+* generated files
+- ignored by version control
+```
+
 ## Contributing
 
 Bug reports and pull requests are welcome on [Disroot][disroot].