Created Sc2004 (markdown)

Sc2004.md

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+# Don't use $ on variables in (( )).
+
+### Problematic code:
+
+    echo $(($n+1))
+
+### Correct code:
+
+    echo $((n+1))
+
+### Rationale:
+
+The `$` on regular variables in arithmetic contexts has no purpose, and is just noise. 
+
+The `$` does matter for special variables like `$1` vs `1`, `$#` vs `#`, and ShellCheck does not produce this style suggestion for these cases.