LICENSE: move the LICENSE file back to the original place

[kennytm]

pkg.go.dev requires a LICENSE file in order to view the docs.

However, in #1 the original LICENSE file is renamed to check.v1_license without a proper replacement. This makes the module missing on pkg.go.dev:

https://pkg.go.dev/github.com/pingcap/[email protected]

Since godoc.org is now a perma-redirect to pkg.go.dev, we must need the LICENSE file in place.

[kennytm]

PTAL @zimulala