add purchase_rappel. manage rappel commission for vendors

purchase_rappel/readme/CONFIGURE.md

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+## Company Settings
+
+Go to **Purchase > Configuration > Settings > Invoicing** and set:
+
+- **Rappel Journal**: Sales journal where rappel commission invoices will be posted.
+- **Commission Product**: Product used as invoice line in rappel commission invoices (should have an income account and taxes configured).
+
+## Vendor Configuration
+
+Open a vendor contact and go to the **Sales & Purchases** tab:
+
+- Enable **Rappel Commission**.
+- Set the **Commission Rate (%)** (e.g., 5.00).