Use absolute link in README

which is included on PyPI where a relative one wouldn't work.

Author: lagru

README.md

@@ -16,7 +16,7 @@ It does so by supporting widely used readable conventions such as `array of dtyp
 
 ## Installation & getting started
 
-Please refer to the [user guide](docs/user_guide.md) to get started with docstub.
+Please refer to the [user guide](https://github.com/scientific-python/docstub/blob/main/docs/user_guide.md) to get started with docstub.
 
 
 ## Contributing