fix(doc): Add a test on README example (refs #333)

lettre_email/tests/skeptic.rs

@@ -7,6 +7,14 @@ use std::process::Command;
 #[test]
 fn book_test() {
     let mut book_path = env::current_dir().unwrap();
+    let readme = Path::new(file!())
+            .parent()
+            .unwrap()
+            .parent()
+            .unwrap()
+            .parent()
+            .unwrap()
+            .join("../README.md");
     book_path.push(
         Path::new(file!())
             .parent()
@@ -21,6 +29,7 @@ fn book_test() {
     for md in glob(&format!("{}/*.md", book_path.to_str().unwrap())).unwrap() {
         skeptic_test(&md.unwrap());
     }
+    skeptic_test(&readme);
 }
 
 fn skeptic_test(path: &Path) {