fixed #235 - shell variables have to be exported to be accessible in subshells

README.md

@@ -32,13 +32,13 @@ source repository and the installation directory. To change the
 installation directory:
 
 ```bash
-$ PRELUDE_INSTALL_DIR="$HOME/.emacs.d" && curl -L https://github.com/bbatsov/prelude/raw/master/utils/installer.sh | sh
+$ export PRELUDE_INSTALL_DIR="$HOME/.emacs.d" && curl -L https://github.com/bbatsov/prelude/raw/master/utils/installer.sh | sh
 ```
 
 To change the source repository:
 
 ```bash
-$ PRELUDE_URL="https://github.com/yourname/prelude.git" && curl -L https://github.com/bbatsov/prelude/raw/master/utils/installer.sh | sh
+$ export PRELUDE_URL="https://github.com/yourname/prelude.git" && curl -L https://github.com/bbatsov/prelude/raw/master/utils/installer.sh | sh
 ```
 
 Note that the installer will back up any existing `.emacs` file or