Created SC2321 (markdown)

SC2321.md

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+## Array indices are already arithmetic contexts. Prefer removing the $(( and )).
+
+### Problematic code:
+
+```sh
+values[$((i+1))]=1
+```
+
+### Correct code:
+
+```sh
+values[i+1]=1
+```
+### Rationale:
+
+In indexed arrays (but not associative ones), the array index is already an arithmetic context. There is no point or value in wrapping it in an additional, explicit `$((..))`.
+
+### Exceptions:
+
+If ShellCheck has failed to realize that your array is associative, or if you for stylistic reasons prefer the redundancy, you can ignore this message. 
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!