add README.md

README.md

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+To implement the confirmation, add this to the composer.json of your module:
+
+```
+  "require": {
+    ...,
+    "d3/confirmComposerPackage": "^1.0"
+  },
+  "extra": {
+    "packageConfirmation": {
+      "question": "your confirmation question (y/N) ",
+      "acceptedanswers": [
+        "y",
+        "yes"
+      ]
+    }
+  },
+```