chore(env): clarify default value mapping for `SUBREDDIT`

.env.template

@@ -16,7 +16,8 @@ RANDOM_THREAD=""
 # Valid options are "light" and "dark". Default: "light"
 THEME=""
 
-# Enter a subreddit, e.g. "AskReddit".
+# Enter a subreddit, e.g. "AskReddit". If this isn't defined, you'll be asked when
+# the programme runs. Simply press Enter if you wish to use the default "AskReddit" subreddit when prompted.
 SUBREDDIT=""
 
 # Filters the comments by range of lenght (min and max characters)