add a note about testing in main.c

c/Makefile

@@ -2,7 +2,7 @@ NAME=blake3
 CC=gcc
 CFLAGS=-O3 -Wall -Wextra -std=c11 -pedantic
 TARGETS=
-EXTRAFLAGS=-DBLAKE3_TESTING
+EXTRAFLAGS=
 
 ifdef BLAKE3_NO_SSE41
 EXTRAFLAGS += -DBLAKE3_NO_SSE41

c/main.c

@@ -1,3 +1,9 @@
+/*
+ * This main file is intended for testing via `make test`. It does not build in
+ * other settings. See README.md in this directory for examples of how to build
+ * C code.
+ */
+
 #include <assert.h>
 #include <errno.h>
 #include <stdbool.h>