tests: add shellchecks and shfmt to README (#2458)

See https://github.com/FreshRSS/FreshRSS/pull/2454#discussion_r306174996

tests/README.md

@@ -5,3 +5,7 @@ cd ./tests/
 wget https://phar.phpunit.de/phpunit.phar
 php phpunit.phar --bootstrap bootstrap.php
 ```
+
+The `shellchecks.sh` script is used to safeguard shell scripts from common
+shell script bugs and to ensure a consistent style.
+It requires [ShellCheck](https://www.shellcheck.net/) and [shfmt](https://github.com/mvdan/sh).