diff --git a/README.md b/README.md
index fd508f9..8a777ce 100644
--- a/README.md
+++ b/README.md
@@ -134,6 +134,8 @@ Everything that changes what gets built belongs in the variant id, since it name
 
 `crafter-build clean` removes the project's `bin/` and `build/` trees. It doesn't load `project.cpp`, so it still works when the project no longer compiles.
 
+A build step fails when its command exits non-zero — never because it printed something. Warnings are printed to stderr and the build carries on; nothing here implies `-Werror`, so add it to `cfg.compileFlags` if you want warnings to be fatal, and they will then be fatal every time. Note that a warning only appears when the unit it belongs to is actually recompiled, so a build with nothing to do is silent.
+
 ## Tests
 
 Tests live under `tests/<Name>/`. The simplest case is a single C++ file: