Update README.md instructions for contributions

Author: alandefreitas

README.md

@@ -2500,6 +2500,8 @@ If contributing with code, please leave the OpenGL backend and pedantic mode ON
 
 </details>
 
+If contributing to the documentation, please edit [`README.md`](README.md) directly, as the files in [`./docs`](./docs) are automatically generated with [mdsplit](https://github.com/alandefreitas/mdsplit).
+
 ### Contributors
 
 <!-- readme: collaborators,contributors,ankane -start -->