docs: add README to custom_strings directory

Author: d-Rickyy-b

blackjackbot/lang/custom_strings/README.md

@@ -0,0 +1,9 @@
+## custom_strings directory
+
+This directory was created to add translations on the fly, when the bot runs inside a docker container.
+
+```
+└───lang
+    ├───custom_strings  // custom translations
+    └───strings         // translations provided by the package maintainer
+```