📝 Improve starter documentation

[babakamyljanovssw]

💫 Describe the Issue With the Current Docs

I've used this link https://tina.io/docs/introduction/using-starter to get started with TinaCMS.
After I created the app, I ran "yarn dev" as per documentation. But it has failed because terminal didn't recognize "tinacms". I needed to run "yarn install" first then run app to resolve this issue.

💼 Tasks

• Add Run yarn install step before Run yarn dev to documentation

📸 Screenshots

Figure: Add yarn install step here