Add more comment notes about customisation. [skip ci]

_recipes.py

@@ -1,3 +1,10 @@
+# --------------------------------------------------------------------
+# This file defines default recipes distributed with newsrack.
+# To customise your own instance, do not modify this file.
+# Add your recipes to _recipes_custom.py instead and new recipe source
+# files to recipes_custom/.
+# --------------------------------------------------------------------
+
 from _recipe_utils import Recipe, onlyon_days, onlyat_hours, onlyon_weekdays
 
 # Only mobi work as periodicals on the Kindle

_recipes_custom.py

@@ -3,6 +3,8 @@
 from _recipe_utils import Recipe
 
 # Define your custom recipes list here
+# Example: https://github.com/ping/newsrack-fork-test/blob/custom/_recipes_custom.py
+
 recipes: List[Recipe] = [
     # Recipe(
     #     recipe="example",

recipes/README.txt

@@ -0,0 +1,4 @@
+This folder holds the default recipes distributed with newsrack.
+
+Do not add your custom recipes to this folder.
+Put them in recipes_custom/ instead.

recipes_custom/README.txt

@@ -1 +1 @@
-Place your custom recipes (*.recipe.py) source files in this folder
+Place your custom recipes (*.recipe.py or *.recipe) source files in this folder