Created API Upgrade (markdown)

API-Upgrade.md

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+# Setup
+
+This API is only used for the upgrading the server.
+
+## Check if update is available
+`GET /update`
+
+### Response
+| code | result |
+| --- | --- |
+| 200 | Everything was successful|
+
+```json
+{
+  "updateRequired": true,
+  "currentVersion": 4,
+  "targetVersion": 6
+}
+```
+
+The version numbers represent the internal db version used. They are only present if `updateRequired` is `true`.
+
+## Update server
+`POST /update`
+
+### Response
+| code | result |
+| --- | --- |
+| 204 | Everything was successful |
+| 500 | An error occured while upgrading |
+
+If an error occurred a message is returned.
+
+```json
+{
+  "error": "Error message!"
+}
+```