Mention install.sh in README.md (holman#232)

* Mention install.sh in README.md

* Cope with the feedback. Add additional explanation for install.sh.

* Apply feedback

README.md

@@ -40,7 +40,8 @@ There's a few special files in the hierarchy.
   expected to setup `$PATH` or similar.
 - **topic/completion.zsh**: Any file named `completion.zsh` is loaded
   last and is expected to setup autocomplete.
-- **topic/\*.symlink**: Any files ending in `*.symlink` get symlinked into
+- **topic/install.sh**: Any file named `install.sh` is executed when you run `script/install`. To avoid being loaded automatically, its extension is `.sh`, not `.zsh`.
+- **topic/\*.symlink**: Any file ending in `*.symlink` gets symlinked into
   your `$HOME`. This is so you can keep all of those versioned in your dotfiles
   but still keep those autoloaded files in your home directory. These get
   symlinked in when you run `script/bootstrap`.