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 +1,11 @@
+{
+  "indicators": {
+    "git_repo": { "enabled": true, "emblem": "emblem-repo-git" },
+    "claude_dir": { "enabled": true, "emblem": "emblem-repo-claude", "marker_directory": ".claude" },
+    "uncommitted_changes": { "enabled": true, "emblem": "emblem-repo-dirty" },
+    "unpushed_commits": { "enabled": true, "emblem": "emblem-repo-unpushed" },
+
+    "node_project": { "enabled": false, "emblem": "emblem-important", "marker_file": "package.json" },
+    "virtualenv": { "enabled": false, "emblem": "emblem-symbolic-link", "marker_directory": ".venv" }
+  }
+}