Clean up function snippet added with tests.

Snippet for clean script exit with parameter for exit code. Updated description in readme.

README.md

@@ -13,3 +13,17 @@ function call:
 check_folder_for_extension ./cffe_test "*.jpg *.JPG"
 ```
 For multiple extensions (or other parts of the file name to match) space separate the list, like shown above.
+
+## Clean up
+
+Cleaning up in a script and exit with a given exit code.
+
+function call:
+```
+clean_up 0;
+```
+
+For testing the function call the script with:
+```
+clean_up.sh test
+```