Change --save-dev to --save in documentation

Assemble plugins are looked up in `dependencies` of `package.json`, not
in `devDependencies`. So user of plugins should install them as that.

Following current instructions and using `--save-dev` leads to plugin
not working, because it's not called.
Merge pull request #17 from mostafah/doc-update

Change --save-dev to --save in documentation