Update README.md

README.md

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+![pdf version](https://img.shields.io/bower/v/adminlte.svg)
 # Bitcoin_Ethereum_Blockchain_docs
 Collection of useful documents about Bitcoin, Ethereum and other blockchain technologies.
 ## To contribute
-To add a document to this collection please open a new pull request with a url of the proposed document.
+Contribution are always welcome and recommended! Here is how:
+
+Fork the repository (here is the guide).
+Clone to your machine git clone https://github.com/YOUR_USERNAME/Blockchain.git
+Make your changes
+Create a pull request