add usage examples

closes #1

examples/exec-krm/README.md

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+# Exec Plugin Example
+
+This example runs `YqTransformer` as an **exec KRM function**.
+
+## Locate the plugin executable
+
+The `config.kubernetes.io/function` annotation in `yq-transformer.yaml`
+points to the plugin's executable path in the `exec.path` key.
+This path is relative to `kustomization.yaml` and can be anywhere on disk.
+
+## Build with alpha plugins and exec enabled
+
+```bash
+kustomize build --enable-alpha-plugins --enable-exec .
+```
+
+You should see a label `environment: dev` added to one of the Deployments.

examples/exec-krm/kustomization.yaml

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+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - ../manifests
+
+transformers:
+  - yq-transformer.yaml

examples/exec-krm/yq-transformer.yaml

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+apiVersion: kustomize.bdeshi.space/v1
+kind: YqTransformer
+metadata:
+  name: yq-transformer-dev-label # this can be anything
+  annotations:
+    config.kubernetes.io/function: |
+      exec:
+        path: ../../YqTransformer.sh
+targets:
+  - apiVersion: apps/v1
+    kind: Deployment
+    spec:
+      template:
+        spec:
+          serviceAccountName: dev-app
+expression: |
+  .metadata.labels.environment = "dev"
+  | .spec.template.metadata.labels.environment = "dev"