Add a visible notice in the README (sourcegraph#64542)

![CleanShot 2024-08-22 at 17 28
24@2x](https://github.com/user-attachments/assets/7c694eae-b14c-41bb-a26b-099d775d7c0a)

Author: jhchabran

README.md

@@ -1,3 +1,12 @@
+> [!NOTE]
+> Sourcegraph transitioned to a private monorepo. This repository, `sourcegraph/sourcegraph-public-snapshot` is a publicly available copy of the `sourcegraph/sourcegraph` repository as it was just before the migration.
+
+> [!TIP]
+> If you are interested in working with the code, this [commit](https://github.com/sourcegraph/sourcegraph-public-snapshot/commit/1cd36d2dbbd2a9ab638cc437d208d2717eaefb0b) is the last one made under an Apache License.
+
+---
+
+
 <p align="center">
 <a href="https://sourcegraph.com/" target="_blank">
 <picture>