Add ERPNext DocType rule examples to README

[abdoruzaqi]

Motivation

• Provide concrete, end-to-end examples showing how FlexiRule maps to common ERPNext DocTypes to help users adopt the app quickly.
• Illustrate typical Rule configurations (trigger, condition tree, actions) for real workflows such as credit checks, approvals, stock protections, and leave automation.

Description

• Added a new "ERPNext DocType Rule Examples" section to README.md containing five complete examples for Sales Invoice, Purchase Order, Stock Entry, Leave Application, and Sales Order.
• Each example documents the Rule trigger, Condition Tree, Actions (graph steps), and expected Result to demonstrate how to wire Rules, Conditions, and Actions together.
• Examples show common actions such as Fetch Customer Credit Limit, Raise Error, Set Value, and Notify to map visual nodes to ERPNext behavior.
• This is a documentation-only change confined to README.md and does not modify runtime code.

Testing

• No automated tests were executed because this is a documentation-only change.
• Documentation build or linter checks were not run as part of this change.

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Codex Task