Created SC1144 (markdown)

SC1144.md

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+## external-sources can only be enabled in .shellcheckrc, not in individual files.
+
+### Problematic code:
+
+```sh
+# shellcheck external-sources=true
+source /dev/zero
+```
+
+### Correct code:
+
+Add `external-sources=true` to `.shellcheckrc`
+
+### Rationale:
+
+Due to its origins as an online tool, ShellCheck will by default run in a sandbox where it only reads the files explicitly named on the command line.
+
+The `external-sources` [[directive]] allows disabling this, but must be specified in `.shellcheckrc`. This is because the sandbox would be useless if the sandboxed script can disable it for itself.
+
+### Exceptions:
+
+None.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!