extensions: make note of default priority

Author: dconrad5

docs/extensions.md

@@ -14,6 +14,8 @@ As an example, if you wished to change "20_audio_converter.py" to "50_audio_conv
 
 This would also work if you wished to create multiple instances of the same extension. For example, if you wished to have a "command_on_download.py" which runs early and one which runs late, you could create two symlinks in the user extension directory, "10_command_on_download.py" and "80_command_on_download_late.py".
 
+Extensions with no priority prefix automatically get priority 99. Therefore if an extension needs to run later than everything, use a value of 100 or greater.
+
 ### Default Extensions Included With gPodder
 
 The following extensions are included with gPodder 3: