fix msvc

vendor/mruby-compiler2/README.md

@@ -0,0 +1,20 @@
+# mruby-compiler2
+
+New mruby compiler based on Prism.
+
+## install by mrbgems
+
+- add conf.gem line to `build_config.rb`
+
+```ruby
+MRuby::Build.new do |conf|
+
+    # ... (snip) ...
+
+    conf.gem :github => 'picoruby/mruby-compiler2'
+end
+```
+
+## License
+under the MIT License:
+- see LICENSE file