Merge pull request #11487 from ethereum/readme-socials

[Readme] Add follow Solidity on Mastodon to socials as Twitter alternative

Author: chriseth

README.md

@@ -4,6 +4,7 @@
 [![Gitter Chat](https://img.shields.io/badge/Gitter%20-chat-brightgreen?style=plastic&logo=gitter)](https://gitter.im/ethereum/solidity)
 [![Solidity Forum](https://img.shields.io/badge/Solidity_Forum%20-discuss-brightgreen?style=plastic&logo=discourse)](https://forum.soliditylang.org/)
 [![Twitter Follow](https://img.shields.io/twitter/follow/solidity_lang?style=plastic&logo=twitter)](https://twitter.com/solidity_lang)
+[![Mastodon Follow](https://img.shields.io/mastodon/follow/000335908?domain=https%3A%2F%2Ffosstodon.org%2F&logo=mastodon&style=plastic)](https://fosstodon.org/@solidity)
 
 You can talk to us on Gitter and Matrix, tweet at us on Twitter or create a new topic in the Solidity forum. Questions, feedback, and suggestions are welcome!