Added alternative of using a ShellCheck directive. Tweaked language.

SC2148.md

@@ -1,10 +1,16 @@
-Add shebang to the top of your script:
+Add a shebang line to the top of your script:
 
 ```bash
 #!/bin/bash
 ```
 
-Alternatively, if that is not possible / desirable, you can use the `--shell` switch:
+Or, for scripts that will not be executed (*e.g.*, `~/.bashrc`), use a directive:
+
+```bash
+# shellcheck shell=sh
+```
+
+If neither of those options are possible or desirable, you can invoke ShellCheck with the `--shell` switch:
 ```bash
 shellcheck --shell=bash without-shebang.sh
 ```