Added test and test description to check_folder_for_extension function.

README.md

@@ -1,2 +1,13 @@
 Collection of useful bash functions.
 
+Every script file includes the function and a test to show how it's meant to be used and check that the function really works. Just executing the script runs that test.
+
+## Check folder for extension
+
+Checks a given folder for certain file extensions recursively. The [check_folder_for_extension test folder](cffe_test) contains of 13 files, 12 of them images, which should be found by the test.
+
+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.