add DevKit instructions to README

kojix2 · kojix2 · commit 860f0014ac91 · 2019-02-21T10:40:07.000+09:00
#157 Reported by Matt Steen. Thanks!
diff --git a/README.md b/README.md
@@ -43,6 +43,7 @@ iruby register --force
 
 ### Windows
 Install git and Jupyter with [Anaconda](https://www.anaconda.com/) (recommended). 
+[DevKit](https://rubyinstaller.org/add-ons/devkit.html) is necessary for building RubyGems with native C-based extensions.
 
 ```shell
 gem install cztop
