Readme: use relative link to docs

Relative links work well locally in text editors and on github, whereas
an absolute link like this can be mildly frustrating when following from
a text editor.

dotcom-rendering/README.md

@@ -139,7 +139,7 @@ If you get lint errors, you can attempt to automatically fix them with:
 $ make fix
 ```
 
-See [the makefile](https://github.com/guardian/dotcom-rendering/blob/main/dotcom-rendering/makefile) for the full list.
+See [the makefile](./makefile) for the full list.
 
 [Read about testing tools and testing strategy](docs/testing.md).