Merge pull request #425 from deikatsuo/patch-1

Update README.md

README.md

@@ -149,6 +149,7 @@ See also [Friends of Rust](https://www.rust-lang.org/friends.html) (organization
 * **Operating systems**, see also [A comparison of operating systems written in Rust](https://github.com/flosse/rust-os-comparison)
   * [redox-os/redox](https://github.com/redox-os/redox) — [<img   src="https://api.travis-ci.org/redox-os/redox.svg?branch=master">](https://travis-ci.org/redox-os/redox)
   * [thepowersgang/rust_os](https://github.com/thepowersgang/rust_os) — [<img src="https://api.travis-ci.org/thepowersgang/rust_os.svg?branch=master">](https://travis-ci.org/thepowersgang/rust_os)
+  * [helena-project/tock](https://github.com/helena-project/tock) — A secure embedded operating system for Cortex-M based microcontrollers
 * **System tools**
   * [Aaronepower/tokei](https://github.com/Aaronepower/tokei) — counts the lines of code [<img src="https://img.shields.io/travis/Aaronepower/tokei.svg">](https://travis-ci.org/Aaronepower/tokei)
   * [buster/rrun](https://github.com/buster/rrun) — a command launcher for Linux, similar to gmrun  [<img src="https://api.travis-ci.org/buster/rrun.svg?branch=master">](https://travis-ci.org/buster/rrun)