Update library version in README.md

Author: ishantanu

README.md

@@ -50,7 +50,7 @@ async fn main() {
 
 1. Add to your `Cargo.toml`:
     ```toml
-    octopust = "0.3.0"
+    octopust = "0.4.0"
     ```
 
 2. See the [examples/](examples/) directory for more usage patterns.
@@ -71,4 +71,4 @@ MIT
 ---
 
 *octopust* is not affiliated with Octopus Energy.  
-Logo and trademark rights belong to their respective owners.
+Logo and trademark rights belong to their respective owners.