Add Discord badge with invite link

Author: wcoenraads

README.md

@@ -4,6 +4,7 @@
     <div>
         <a href="https://crates.io/crates/s2energy"><img src="https://img.shields.io/crates/v/s2energy" /></a>
         <a href="https://docs.rs/s2energy"><img src="https://img.shields.io/docsrs/s2energy" /></a>
+        <a href="https://discord.com/invite/NyFMEPmuDw"><img src="https://img.shields.io/discord/1351281839913832510"></a>
     </div>
 </div>
 <br />
@@ -19,4 +20,4 @@ This crate provides Rust types for all types specified by S2. It also provides u
 JSON over WebSockets is a common and recommended way to implement S2, but you're free to choose a different format and communication protocol. In that case, the types in this crate should still be useful but you may wish to disable the `websockets-json` feature.
 
 ## Documentation
-You can find the crate documentation at [docs.rs](https://docs.rs/s2energy). The crate documentation assumes that you are familiar with S2; if this is not the case, it may be useful to refer to [the S2 documentation website](https://docs.s2standard.org/docs/welcome/). That documentation explains S2 concepts in more detail, and contains a reference of all messages and types in the S2 specification.
+You can find the crate documentation at [docs.rs](https://docs.rs/s2energy). The crate documentation assumes that you are familiar with S2; if this is not the case, it may be useful to refer to [the S2 documentation website](https://docs.s2standard.org/docs/welcome/). That documentation explains S2 concepts in more detail, and contains a reference of all messages and types in the S2 specification.