doc: Add instructions to run GNU tests

DEVELOPER_INSTRUCTIONS.md

@@ -9,6 +9,19 @@ The documentation is updated everyday on this repository:
 
 https://github.com/uutils/coreutils-docs
 
+Running GNU tests
+-----------------
+
+- Check out https://github.com/coreutils/coreutils next to your fork as gnu
+- Check out https://github.com/coreutils/gnulib next to your fork as gnulib
+- Rename the checkout of your fork to uutils
+
+At the end you should have uutils, gnu and gnulib checked out next to each other.
+
+- Run `cd uutils && ./util/build-gnu.sh && cd ..` to get everything ready (this may take a while)
+- Finally, you can run `tests with bash uutils/util/run-gnu-test.sh <test>`. Instead of `<test>` insert the test you want to run, e.g. `tests/misc/wc-proc`.
+
+
 Code Coverage Report Generation
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