📗 Add Travis's Build Status Badge

NOTE: this requires the repo's owner to turn on the repo's CI
mannually. Please kindly go to https://travis-ci.org/.

cc @wayslog

Author: arrowrowe

README.md

@@ -1,4 +1,10 @@
 # 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。