add back build status blank

wayslog · wayslog · commit dac46df2d52d · 2016-05-05T01:31:53.000+08:00
diff --git a/README.md b/README.md
@@ -1,5 +1,9 @@
 # RustPrimer
 
+[![build status][build-badge]][build-url]
+
+[build-badge]: https://api.travis-ci.org/rustcc/RustPrimer.svg
+[build-url]: https://travis-ci.org/rustcc/RustPrimer
 
 The Rust primer for beginners.
 给初学者的Rust中文教程。
