Add clarity about installation to README

[BoDonkey]

Summary

Summarize the changes briefly, including which issue/ticket this resolves. If it closes an existing Github issue, include "Closes #[issue number]"
This PR clarifys how this repo should be used to stave off developers trying to install the starter kits into existing projects. Partially closes PRO-4870

What are the specific steps to test this change?

For example:

1. Run the website and log in as an admin
2. Open a piece manager modal and select several pieces
3. Click the "Archive" button on the top left of the manager and confirm that it should proceed
4. Check that all pieces have been archived properly

1. Read the README.md

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-4870 Our starter kits need clear instructions for those who come right to the README

Many people will discover our starter kits by going directly to the repos. The README files don't currently start with an explanation of how to correctly use them. Even more basic, they should start by saying "this is a starter kit for your own projects. It's not a package you install."

We're seeing confusion about this from at least two customers / customer candidates in the pipeline.

• This is a starter kit you can copy to make your own project
• The super easy way is to use our CLI
• Or, fork this repo under a new name in your own github to create your own project
• Don't npm install this
• "Also known as a boilerplate…"