docs: add README with install, configuration and migration

Written for someone who has never seen the pre-split extension: what the plugin adds to Nemo, what it costs, every config key, and the exact steps to move over from the single file version or go back.

config.example.json

@@ -0,0 +1,7 @@
+{
+  "cache_ttl_seconds": 5,
+  "columns": {
+    "branch": true,
+    "status": true
+  }
+}