Add ETH donation badge

docs/index.md

@@ -7,6 +7,7 @@ description: This is an open source project for validating Solidity code.
 ---
 
 ## Solhint Project
+[![Donate with Ethereum](https://en.cryptobadges.io/badge/micro/0xe8cdf02efd8ab0a490d7b2cb13553389c9bc932e)](https://en.cryptobadges.io/donate/0xe8cdf02efd8ab0a490d7b2cb13553389c9bc932e)
 
 [![Gitter chat](https://badges.gitter.im/gitterHQ/gitter.svg)](https://gitter.im/solhint/Lobby)
 [![Build Status](https://travis-ci.org/protofire/solhint.svg?branch=master)](https://travis-ci.org/protofire/solhint)
@@ -111,3 +112,8 @@ See EsLint docs about formatters [here](https://eslint.org/docs/user-guide/forma
 ### Licence
 
 MIT
+
+## Back us
+Solhint is free to use and open-sourced. If you value our effort and feel like helping us to keep pushing this tool forward, you can send us a small donation. We'll highly appreciate it :)
+
+[![Donate with Ethereum](https://en.cryptobadges.io/badge/micro/0xe8cdf02efd8ab0a490d7b2cb13553389c9bc932e)](https://en.cryptobadges.io/donate/0xe8cdf02efd8ab0a490d7b2cb13553389c9bc932e)