Document config re toot and alt-text templates

Fix template behavior when comment is `None`
(not relevant in our use case, but may be important one day).

`example.json` now contains a single image
(reference for available template fields).

bloomscroller.py

     mastodon = Mastodon(access_token="secrets.txt")
     image = get_next_image(get_images(config.ALBUM_API), get_previous_id())
     if image:
+        if image['comment'] is None:
+            image['comment'] = ""  # Better not mess up templates when no comment.
         media = mastodon.media_post(
             requests.get(
                 image["derivatives"][config.MEDIA_SIZE]["url"]).content,

config.py

 
 ALBUM_API = "https://photos.nandn.org.il/piwigo/ws.php?format=json&method=pwg.categories.getImages&cat_id=21&per_page=529"
 
+# Both templates can use image fields like {name} and {comment} (see example.json)
+# I don't need it in my use case, but go ahead
 TOOT_TEMPLATE = "#Bloomscrolling"
 ALT_TEXT_TEMPLATE = "Flowers from n&n's garden arranged in a vase.\nPhotographed at {date_creation}."
+
 TOOT_LANGUAGE = "en"
 TOOT_VISIBILITY = "public"
 MEDIA_SIZE = "4xlarge"