Created SC1091 (markdown)

SC1091.md

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+## Not following: some reason here.
+
+Reasons include: file not found, no permissions, not included on the command line.
+
+### Problematic code:
+
+    source somefile
+
+### Correct code:
+
+    # shellcheck disable=SC1091
+    source somefile
+
+### Rationale:
+
+ShellCheck, for whichever reason, is not able to access the source file.
+
+This could be because you did not include it on the command line, did not use `shellcheck -x` to allow following other files, don't have permissions or a variety of other problems.
+
+Feel free to ignore the error with a [[directive]].
+
+### Exceptions:
+
+If you're fine with it, ignore the message with a [[directive]].