document build on save in helix config guide

Author: Techatrix

content/zls/editors/helix.smd

@@ -26,6 +26,12 @@ command = "/path/to/zls_executable"
 # Further information on how to configure ZLS:
 # https://zigtools.org/zls/configure/
 
+# Whether to enable build-on-save diagnostics
+#
+# Further information about build-on save:
+# https://zigtools.org/zls/guides/build-on-save/
+# config.enable_build_on_save = true
+
 # omit the following line if `zig` is in your PATH
 config.zig_exe_path = "/path/to/zig_executable"
 ```