refactor: ad migration guide (planning#1684))

docs/update-guide.md

+## Update from 1.0.0 to 2.0.0
+
+### Rename
+- [ ] Rename the static client getters in ClientFactory from ``getClient`` to `createClient` for:
+  - `ClientFactory.createSenderClient(config)`
+  - `ClientFactory.createSubscriberClient(config)`
+  - `ClientFactory.createRouterClient(config)`
+- [ ] Rename the static getters in ReplyChannel from ``from`` to `to` for:
+  - `ReplyChannel.ofEmail(...)`
+  - `ReplyChannel.ofDemail(...)`
+  - `ReplyChannel.ofFink()`
+  - `ReplyChannel.ofElster()`
+  - `ReplyChannel.ofFitConnect()`
+
+### Fix imports
+Several model classes have been moved to the API module. Therefore, the imports of the following classes have to be adjusted:
+
+- [ ] Received Submission
+
+### Configuration
+
+#### ProxyConfig
+
+#### Local Data Schemas
+
+### Deprecations
+
+