CONTRIBUTING: mention that EXIT_ codes should be used

CONTRIBUTING.md

@@ -74,6 +74,7 @@ In addition, libsecp256k1 tries to maintain the following coding conventions:
 * User-facing comment lines in headers should be limited to 80 chars if possible.
 * All identifiers in file scope should start with `secp256k1_`.
 * Avoid trailing whitespace.
+* Use the constants `EXIT_SUCCESS`/`EXIT_FAILURE` (defined in `stdlib.h`) to indicate program execution status for examples and other binaries.
 
 ### Tests