2.9 docs - fix broken link (chartjs#8241)

docs/README.md

@@ -56,7 +56,7 @@ var myChart = new Chart(ctx, {
 
 ## Contributing
 
-Before submitting an issue or a pull request to the project, please take a moment to look over the [contributing guidelines](https://github.com/chartjs/Chart.js/blob/master/docs/developers/contributing.md) first.
+Before submitting an issue or a pull request to the project, please take a moment to look over the [contributing guidelines](./developers/contributing.md) first.
 
 For support using Chart.js, please post questions with the [`chartjs` tag on Stack Overflow](https://stackoverflow.com/questions/tagged/chartjs).