Make config.php settings also available as environment variables (closes #118)

config-dist.php

@@ -1,5 +1,19 @@
 <?php
 
+# Settings can also be overwritten in two ways
+#
+# First priority
+# A .txt file with the same name as the setting in /data/settingoverrides
+# the content of the file is used as the setting value
+#
+# Second priority
+# An environment variable with the same name as the setting and prefix "GROCY_"
+# so for example "GROCY_BASE_URL"
+#
+# Third priority
+# The settings defined here below
+
+
 # Either "production", "dev" or "prerelease"
 Setting('MODE', 'production');