docs: add a requirements section to the developer docs.

Add a section on installing sed, which differs between GNU and OSX.

docs/developer/requirements.md

@@ -0,0 +1,17 @@
+# Requirements
+
+Depending on your setup, you might need different tools to develop on argos. We try to list them here.
+
+## Mac OS
+
+### Gnu Sed
+
+You will need to have gnu-sed installed. "sed" installed by default on MacOS machines differ from the gnu version (It's used for some post-documentation-generation hooks)
+
+```bash
+
+brew install gnu-sed
+
+# This will explain how to add it to your path (to replace the default one)
+brew info gnu-sed
+```

docs/index.md

@@ -56,6 +56,7 @@ checks
 ```{toctree}
 :caption: Developer docs
 :hidden:
+developer/requirements
 developer/overview
 developer/dependencies
 developer/new-check