document new rsync options

related to https://github.com/lektor/lektor/pull/707

content/docs/deployment/rsync/contents.lr

@@ -37,3 +37,34 @@ private key without newlines or whitespace.  To find out which type your
 key is look at the first line of the key marker.  For instance `BEGIN EC
 PRIVATE KEY` indicates an `EC` key.  If no key type is defined `RSA` is
 assumed.
+
+## Deletion Support
+
+To keep two directories truly in sync when deploying with `rsync`,
+it's necessary to explicitly tell `rsync` to remove files or directories
+on target that don't exist on source anymore. This can be done using the
+`?delete` URL parameter:
+
+```ini
+[servers.production]
+target = rsync://server/path/to/folder?delete
+```
+
+If the parameter is provided Lektor will issue a `rsync --delete-delay`,
+which performs deletions after all other transfers ended, and only in case
+there were no failures.
+
+!!!! Note that the `?delete` option will remove **any file or directory**
+on target that does not exist on source. This means that if you have files
+in the target tree that are not managed by lektor, they will get removed.
+
+## Exclusion Support
+
+You can exclude items from being synced by using one or more `exclude`
+parameters. This is also useful in combination with `delete` to prevent
+removal of files in the target tree that are not managed by lektor:
+
+```ini
+[servers.production]
+target = rsync://server/path/to/folder?delete&exclude=target_item_1&exclude=target_item_2
+```