update readme instructions for zsh users

README.md

@@ -2,11 +2,25 @@
 
 > Shell autocompletion for [Yarn](https://github.com/yarnpkg/yarn)
 
-**Note:** bash is currently the only supported shell.
+## Installation
 
-## Usage
+1. Download the completion file to your machine.
 
-See comment at the top of the file.
+```sh
+$ curl -o ~/.yarn-completion https://raw.githubusercontent.com/dsifford/yarn-completion/master/yarn-completion.bash
+```
+
+2. Source the file in your shell `rc` file.
+
+**`bash`**
+```sh
+$ echo -e '\n. ~/.yarn-completion' >> ~/.bashrc
+```
+
+**`zsh`**
+```sh
+$ echo -e '\nautoload -U bashcompinit && bashcompinit\n. ~/.yarn-completion' >> ~/.zshrc
+```
 
 ## License