Make zsh completion work on $fpath.

This allows the file to be placed, e.g., in a system-wide
`site-functions` directory and just work automatically.

shell/README.md

@@ -17,15 +17,17 @@ adding the following snippet:
 
 ### Zsh
 
+Load up the completion by placing the `exercism_completion.zsh` somewhere on
+your `$fpath` as `_exercism`. For example:
+
     mkdir -p ~/.config/exercism
-    mv ../shell/exercism_completion.zsh ~/.config/exercism/exercism_completion.zsh
+    mv ../shell/exercism_completion.zsh ~/.config/exercism/_exercism
 
-Load up the completion in your `.zshrc`, `.zsh_profile` or `.profile` by adding
-the following snippet
+and then add the directory to your `$fpath` in your `.zshrc`, `.zsh_profile` or
+`.profile` before running `compinit`:
 
-    if [ -f ~/.config/exercism/exercism_completion.zsh ]; then
-      source ~/.config/exercism/exercism_completion.zsh
-    fi
+    export fpath=(~/.config/exercism $fpath)
+    autoload -U compinit && compinit
 
 
 #### Oh my Zsh

shell/exercism_completion.zsh

@@ -1,3 +1,5 @@
+#compdef exercism
+
 _exercism() {
     local curcontext="$curcontext" state line
     typeset -A opt_args
@@ -33,4 +35,4 @@ _exercism() {
     esac
 }
 
-compdef '_exercism' exercism
+_exercism "$@"