Add "Join Discord" link to Readme
Welcome to the official Go implementation of IoTeX protocol! IoTeX is building the next generation of the decentralized blockchain protocol for powering real-world information marketplace in a decentralized-yet-scalable way. Refer to IoTeX [whitepaper](https://iotex.io/research/) for details.
+<a href="https://iotex.io/devdiscord" target="_blank">
+ <img src="https://github.com/iotexproject/halogrants/blob/880eea4af074b082a75608c7376bd7a8eaa1ac21/img/btn-discord.svg" height="36px">
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## Get started
### Minimum requirements
