📝 [signer-p12] Add a short README

(#205)

packages/signer-p12/README.md

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+# Signer implementation using PKCS#12 bundle with node-forge
+
+for [![@signpdf](https://raw.githubusercontent.com/vbuch/node-signpdf/master/resources/logo-horizontal.svg?sanitize=true)](https://github.com/vbuch/node-signpdf/)
+
+[![npm version](https://badge.fury.io/js/@signpdf%2Fsigner-p12.svg)](https://badge.fury.io/js/@signpdf%2Fsigner-p12)
+[![Donate to this project using Buy Me A Coffee](https://img.shields.io/badge/buy%20me%20a%20coffee-donate-yellow.svg)](https://buymeacoffee.com/vbuch)
+
+Uses `node-forge` to create a detached signature of a given `Buffer` using a PKCS#12 certificate bundle.
+
+## Usage
+
+See the [typescript.ts example](/packages/examples/src/typescript.ts).
+
+## Notes
+
+* Make sure to have a look at the docs of the [@signpdf family of packages](https://github.com/vbuch/node-signpdf/).
+* Feel free to copy and paste any part of this code. See its defined [Purpose](https://github.com/vbuch/node-signpdf#purpose).