added invoice app in beginner level

Projects/1-Beginner/InvoiceApp.md

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+# Invoice App
+
+**Tier:** 1-Beginner
+
+An invoice is a document that maintains a record of a transaction between 
+a buyer and seller, such as a paper receipt from a store or online record from an e-tailer.
+
+An invoice must state it is an invoice on the face of the bill. It typically has a unique identifier called the invoice number that is useful for internal and external reference. An invoice typically contains contact information for the seller or service provider in case there is an error relating to the billing.
+Invoices are a critical element of accounting internal controls and audits.
+
+
+## User Stories
+
+-   [ ]  User can enter a Customer name, number, bill amount, gst and other taxes into a input box.
+-   [ ] User can select service date.
+-   [ ] user can generate invoice in pdf form and share it with customer.
+-   [ ] User can preview the invoice.
+-   [ ] User can share invoice to customer's mobile number
+
+## Bonus features
+
+-   [ ] User can print invoice, download and share on any platform.
+-   [ ] User can add their personal details.
+
+
+## Example projects
+
+- [Invoice App](https://github.com/Deepshikhayadav/Sagar-Travels.git)
+- [Vue Invoice App](https://github.com/johnkomarnicki/vue-invoice-app.git)