diff --git a/docs/CONTRIBUTING.md b/docs/CONTRIBUTING.md
index 70e5739c..bd982aa7 100644
--- a/docs/CONTRIBUTING.md
+++ b/docs/CONTRIBUTING.md
@@ -225,16 +225,21 @@ Each one is a binary RNG file which is passed to the `rgbgfx_test` program.
 
 ### External projects
 
-Each `*.cfg` file corresponds to one test.
+Each `.cfg` file corresponds to one test.
 Each one defines the parameters for a real external project that builds using RGBDS.
 
 1. Make sure the downstream project supports
-   <code>make <var>&lt;target&gt;</var> RGBDS=<var>&lt;path/to/RGBDS/&gt;</var></code>.
+   <code>make <var>&lt;target&gt;</var> RGBDS=<var>&lt;path/to/rgbds/&gt;</var></code>.
    While the test suite supports any Make target name, only
    [Make](//gnu.org/software/make) is currently supported, and the Makefile must
    support a `RGBDS` variable to use a non-system RGBDS directory.
 2. Create the new `.cfg` file, following the existing ones.
 
+If an external project does not support `make RGBDS=`, or otherwise fails to
+build with current RGBDS, then then you can edit the project to build correctly,
+and create a patch with `git diff` to be committed as
+<code>test/external/patches/<var>&lt;project&gt;</var>.patch</code>.
+
 ## Container images
 
 The CI will