Update contribution agreement link

Signed-off-by: Juan Cruz Viotti <[email protected]>

README.markdown

@@ -14,9 +14,10 @@ Commercial Use
 --------------
 
 This project is governed by the [AGPL-3.0](./LICENSE) copyleft license and our
-[Contribution Agreement](https://www.sourcemeta.com/contributing/). To ensure
-the sustainability of the project, you can freely make use of this software as
-long the outcome is distributed under the same license. Otherwise, you must
-obtain a [commercial license](./LICENSE-COMMERCIAL) that removes such
+[Contribution
+Agreement](https://github.com/sourcemeta/.github/blob/main/CONTRIBUTING.md). To
+ensure the sustainability of the project, you can freely make use of this
+software as long the outcome is distributed under the same license. Otherwise,
+you must obtain a [commercial license](./LICENSE-COMMERCIAL) that removes such
 restrictions. Read more about our licensing approach
 [here](https://www.sourcemeta.com/licensing/).