docs: add repository relocation banner to README

## What

Added an important notice banner at the top of the README informing users
that the repository has moved from `github/contributors` to
`github-community-projects/contributors`, with instructions to update their
git remote.

## Why

Users with existing clones still pointing at the old github/contributors
remote need a clear, visible notification about the move and a copy-paste
command to update their remote URL.

## Notes

- Uses GitHub's `[!IMPORTANT]` alert syntax which only renders on GitHub — won't display as a callout in other markdown renderers

Signed-off-by: jmeridth <jmeridth@gmail.com>

Author: jmeridth

README.md

@@ -1,3 +1,13 @@
+> [!IMPORTANT]
+> This repository has moved from `github/contributors` to `github-community-projects/contributors`.
+> Please update your git remote:
+>
+> ```shell
+> git remote set-url origin https://github.com/github-community-projects/contributors.git
+> ```
+>
+> Note: replace `origin` with the name of your remote if it's different.
+
 # Contributors action
 
 [![Python package](https://github.com/github-community-projects/contributors/actions/workflows/python-ci.yml/badge.svg)](https://github.com/github-community-projects/contributors/actions/workflows/python-ci.yml)