Update index.md

Author: MichaelCurrin

cheatsheets/shell/index.md

@@ -22,3 +22,6 @@ Unix shell guide.
 - [devhints.io/bash](https://devhints.io/bash)
 - [Bash cheatsheets](https://cheatography.com/tag/bash/) on Cheatography.
 - [Shell](https://github.com/MichaelCurrin/learn-to-code/tree/master/en/topics/shell) in Learn to Code project.
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+See also [shellcheck](https://github.com/koalaman/shellcheck/) - as shell script static analysis tool.