added regex for checking a podcast:guid

Signed-off-by: Brian of London <brian@podping.org>

docs/1.0.md

@@ -541,6 +541,10 @@ Using this pattern, podcasts can maintain a consistent identity across the open
 * All podcasts in the Podcast Index have already been assigned a GUID; but if one exists in the RSS feed, that value is canonical.
 * You can programmatically spot a GUID: it is 36 characters long, and contains four hyphen characters.
 * Be aware that Amazon Music also uses UUIDv5 identifiers, which are calculated differently.
+* The following regular expression (regex) will match a GUID:
+```re
+[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}`
+```
 
 ### Parent
 &nbsp; `<channel>`