README license badge points to a missing file on an invalid branch

## Description

The license badge in the repository README links to:

`https://github.com/base/docs/blob/main/LICENSE.md`

That URL currently returns a 404 with “Ref is invalid.” The repository's default branch is `master`, and there does not appear to be a `LICENSE.md` file at the repository root.

## Steps to reproduce

1. Open the repository README.
2. Select the license badge.
3. Observe that GitHub opens `main/LICENSE.md` and returns a 404.

## Expected behavior

The license badge should resolve to the repository's applicable license information.

Could a maintainer confirm where the license file should live and which existing license the badge should reference? I am happy to prepare a follow-up fix once the intended source is confirmed.

Start in the repository README and verify the license badge target against the default branch and repository root. Confirm the intended license source with a maintainer, then update the badge so it resolves to the applicable license information and verify the link no longer returns a 404.

Written by the indexing model from the issue text.