Document how to use the crate with Rust 2018

Fixes #294.

README.md

@@ -50,6 +50,16 @@ pub fn shave_the_yak(yak: &mut Yak) {
 }
 ```
 
+If you use Rust 2018, you can use instead the following code to import the crate macros:
+
+```rust
+use log::{info, trace, warn};
+
+pub fn shave_the_yak(yak: &mut Yak) {
+    // …
+}
+```
+
 ## In executables
 
 In order to produce log output executables have to use a logger implementation compatible with the facade.