add update guide for 2.1.0

UPDATE.md

@@ -1,5 +1,38 @@
 # Updating
 
+## 2.0.x -> 2.1.0
+
+1. pull the update: `git pull`
+
+2. run `pip install -r requirements.txt` again
+
+3. make the following changes in your config.json file:
+
+```diff
+	...
+	"ntfy": {
+		"enabled": false,
+		"host": "https://ntfy.sh",
+		"user": "",
+		"pass": "",
+		"topic": ""
+	},
++	"crosspost": {
++		"fediverse": {
++			"enabled": false,
++			"instance": "",
++			"client": {
++				"id": "",
++				"secret": ""
++			},
++			"token": "",
++			"visibility": "public",
++			"cw": ""
++		}
++	},
+	...
+```
+
 ## 1.7.x -> 2.0.x
 
 ### prerequisites