feat(paywall): extension basics

lnbits/extensions/paywall/README.md

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+<h1>Example Extension</h1>
+<h2>*tagline*</h2>
+This is an example extension to help you organise and build you own.
+
+Try to include an image
+<img src="https://i.imgur.com/9i4xcQB.png">
+
+
+<h2>If your extension has API endpoints, include useful ones here</h2>
+
+<code>curl -H "Content-type: application/json" -X POST https://YOUR-LNBITS/YOUR-EXTENSION/api/v1/EXAMPLE -d '{"amount":"100","memo":"example"}' -H "Grpc-Metadata-macaroon: YOUR_WALLET-ADMIN/INVOICE-KEY"</code>