extension related to https://github.com/Kozea/Radicale/issues/1855

Sharing-Collections.md

@@ -75,6 +75,20 @@ collection: {user}/sharedaddressbook2(/.+)?
 permissions: r
 ```
 
+Related `rights` extension for read-write but forbid-to-delete-collection-itself:
+
+```
+[group1-sharedcalendar1-rw]
+user: USER1
+collection: {user}/sharedcalendar1(/.+)?
+permissions: rwd
+  
+[group1-sharedaddressbook1-rw]
+user: USER2
+collection: {user}/sharedaddressbook1(/.+)?
+permissions: rwd
+```
+
 ## Sharing a collections read-only to public as WebCAL
 
 **ATTENTION: This is a simple and potentially insecure example**