Add a page about how to use shellcheck recursively

Recursiveness.md

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+By default, `shellcheck` does not support a `-r` option. The reason for this, is that there are different matching patterns for different files.
+
+Script could have a `.sh` extension, no extension and have a range of shebang lines (which have there own testing format):
+
+* `#!/bin/bash`
+* `#!/bin/sh`
+* `#!/usr/bin/env bash`
+* `#!/usr/bin/env sh`
+* etc...
+
+The solution for this problem is to use `shellcheck` in combination with the `find` or `grep` command.
+
+## Without exit code
+
+If you're not interested in the exit code of the `shellcheck` command, you can use (a variation) of one of the following commands:
+
+```
+# Scan a complete folder (recursively)
+find path/to/scripts -type f -exec "shellcheck" "--format=gcc" {} \;
+
+# Scan a complete folder (recursively) for .sh files
+find path/to/scripts -type f -name "*.sh" -exec "shellcheck" "--format=gcc" {} \;
+```
+
+## With exit code
+
+If you want to use `shellcheck` in some kind of testing pipeline, you want to know the exit code of the command.
+
+In this case you can test every matched file individually:
+
+```
+# Scan a complete folder (recursively)
+for file in $(find path/to/scripts -type f); do shellcheck --format=gcc $file; done;
+
+# Scan a complete folder (recursively) for .sh files
+for file in $(find path/to/scripts -type f -name "*.sh"); do shellcheck --format=gcc $file; done;
+```
+
+## Finding files to test
+
+Since not all script have a `.sh` extension, you could use the shebang line of files to determine if they need to be testen (and with which format):
+
+```
+for file in $(grep -IRl "#\!\(/usr/bin/env \|/bin/\)sh" --exclude-dir "var" --exclude "*.txt"); do shellcheck --format=gcc --shell=sh $file; done;
+```