Update development section packages/nuxt Readme

[m0ksem]

Now playground can be called with yarn sandbox:nuxt from the root of the project. But, you'll need firstly build packages/nuxt.

So we need to add build:nuxt: yarn workspace nuxt build command to package.json and write about it in readme as well.

[Us3r-gitHub]

I'd like to contribute to open source. What steps can I take to complete this task?

As far i see, there's already script on package.json https://github.com/epicmaxco/vuestic-ui/blob/develop/package.json#L34