Adding a mermaid diagram for the updater flow.

Signed-off-by: Cliff Hill <xlorep@darkhelm.org>

docs/diagrams/updater.mermaid

@@ -0,0 +1,10 @@
+flowchart TD
+    A[Start] --> B[[Update start timestamp & get last updated]]
+    B --> C([Get track from plex])
+    C --> |more tracks| D[[Remove recently played tracks from playlist]]
+    D --> E[[Insert/update track into database]]
+    E --> C
+    C --> |no more tracks| F[[Clean up genres]]
+    F --> G[[Calculate statistics]]
+    G --> H[[Update end timestamp]]
+    H --> I[End]