add homebrew install discription

EN/2_mri_structure.md

@@ -34,6 +34,11 @@ If you use `apt-get` (or `apt`) for package management in your environment, then
 ```
 $ sudo apt-get install git ruby autoconf bison gcc make zlib1g-dev libffi-dev libreadline-dev libgdbm-dev libssl-dev libyaml-dev
 ```
+If you use `homebrew` for package management in your environment, then you can get all dependencies with the following command:
+
+```
+$ brew install git ruby autoconf bison gcc make zlib libffi readline gdbm openssl libyaml
+```
 
 ## Exercise: Clone the MRI source code
 

JA/2_mri_structure.md

@@ -33,6 +33,12 @@ git、ruby、autoconf、bison、gcc (or clang, etc）、make が必須です。
 $ sudo apt-get install git ruby autoconf bison gcc make zlib1g-dev libffi-dev libreadline-dev libgdbm-dev libssl-dev libyaml-dev
 ```
 
+`homebrew` でインストールする場合は下記のコマンドを実行してみてください。
+
+```
+$ brew install git ruby autoconf bison gcc make zlib libffi readline gdbm openssl libyaml
+```
+
 ## 演習: MRI のソースコードを clone
 
 1. `$ mkdir workdir`