Created SC2042 (markdown)

SC2042.md

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+## Use spaces, not commas, to separate loop elements.
+
+### Problematic code:
+
+```sh
+for f in foo,bar,baz
+do
+  echo "$f"
+done
+```
+
+### Correct code:
+
+```sh
+for f in foo bar baz
+do
+  echo "$f"
+done
+```
+### Rationale:
+
+ShellCheck found a `for` loop where the items appeared to be delimited by commas. These will be treated as literal commas. Use spaces instead.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!