update README with info about the `parts` key in `shows.yaml`

README.md

@@ -44,6 +44,11 @@ Gundam 0079:
     - MSG 0079
     - "0079"
     - First Gundam
+MS IGLOO:
+  path: /home/user/media/Gundam Igloo
+  parts:
+    1: The Hidden One Year War
+    2: Apocalypse 0079
 ```
 
 each top-level key is a show title, which will be used in spoiler
@@ -54,3 +59,7 @@ warnings on posts and in image alt text.  each show has two keys:
   the episodes must be directly inside the directory, the bot will not
   traverse subdirectories to find files.
 - `tags`: an optional list of tags to put on posts from this show.
+- `parts`: an optional map of season numbers to season names.
+  by default, if the bot detects episodes from multiple seasons in a directory,
+  it will refer to them as "season [number]" in post CWs and image alt-text.
+  this is a map rather than a list, since show season numbers may be 1-indexed or 0-indexed.