Release v0.5.0: README refresh

- Add code-viewer polish line to the feature list (pinned gutter sized to
  the widest number, line wrap, jump buttons, theme-aware scrollbars)
- Extend mobile preview links (editor, dark and light paste views)
- Clarify PALETTE_MAX_ITEM covers can items and file attachments
- Document PALETTE_DEFAULT_DARK in docker-compose.yml so the compose file
  really covers every env var

docker-compose.yml

@@ -32,10 +32,16 @@ services:
       # Default: 5242880 (5 MiB).
       # PALETTE_MAX_TEXT: "5242880"
 
-      # Max size in bytes of a single can item (file/text inside a can).
+      # Max size in bytes of a single can item (file/text inside a can) or a
+      # paste file attachment.
       # Default: 26214400 (25 MiB).
       # PALETTE_MAX_ITEM: "26214400"
 
+      # Default dark mode for new visitors. Unset = dark on; set to "false",
+      # "0" or "off" to default to light mode. Visitors who toggle dark mode
+      # keep their choice in their browser.
+      # PALETTE_DEFAULT_DARK: "false"
+
       # HMAC secret for password-unlock cookies. Default: random per start,
       # which logs out every unlocked browser session on restart. Set a fixed
       # secret (any random string) to keep unlock sessions across restarts,