Bump the version number in the readme

Author: nagisa

README.md

@@ -24,15 +24,15 @@ Add to Cargo.toml:
 
 ```toml
 [dependencies]
-hedwig = "2"
+hedwig = "3"
 ```
 
 You may also need to enable additional features in order to use the optional publishers or
 validators, like this:
 
 ```toml
 [dependencies]
-hedwig = { version = "2", features = ["google"] }
+hedwig = { version = "3", features = ["google"] }
 ```
 
 ### Usage