add link to nightly rust docs

Author: japaric

content/2019-02-20-newsletter-15.md

@@ -126,7 +126,7 @@ For a bit more of detail you can check the [logs] and [minutes] from two meeting
 [@adamgreig]: https://github.com/adamgreig
 [stm32ral]: https://github.com/adamgreig/stm32ral
 
-- As of `nightly-2019-02-05` the [embedded Rust Book][book] is included as part of the docs shipped with the Rust toolchain!
+- As of `nightly-2019-02-05` the [embedded Rust Book][book] is included as part of the docs shipped with the Rust toolchain! [This is the online version](https://doc.rust-lang.org/nightly/#embedded-systems).
 
 [book]: https://docs.rust-embedded.org/book/index.html