Created SC2219 (markdown)

SC2219.md

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+## Instead of `let expr`, prefer `(( expr ))` .
+
+### Problematic code:
+
+```sh
+let a++
+```
+
+### Correct code:
+
+```sh
+(( a++ ))
+```
+### Rationale:
+
+The `(( .. ))` arithmetic compound command evaluates expressions in the same way as `let`, except it's not subject to glob expansion and therefore requires no additional quoting or escaping. 
+
+This warning only triggers in Bash/Ksh scripts. In Sh/Dash, neither `let` nor `(( .. ))` are defined, but can be simulated with `[ $(( expr )) -ne 0 ]` to retain exit code, or `: $(( expr ))` to ignore it.
+
+### Exceptions:
+
+None.
+
+### More information
+
+* Bash Hacker's Wiki: [The let builtin command](http://wiki.bash-hackers.org/commands/builtin/let)