message: docs: add a blogpost link in External Resources section
  The blogpost is about the usage of tracing and some of its complementary modules.
  Related [Reddit comment]( https://www.reddit.com/r/rust/comments/jv6a5j/productiongrade_logging_in_rust_applications/gcrjoa6?utm_source=share&utm_medium=web2x&context=3).
diff: @@ -439,6 +439,7 @@ Tracing.
  #### Blog Posts
  * [Diagnostics with Tracing][tokio-blog-2019-08] on the Tokio blog, August 2019
  +* [Production-Grade Logging in Rust Applications][production-logging-2020], November 2020
  [tokio-blog-2019-08]: https://tokio.rs/blog/2019-08-tracing/
  @@ -453,6 +454,7 @@ Tracing.
  [rust-conf-2019-08-slides]: https://www.elizas.website/slides/rustconf-8-2019.pdf
  [rusty-days-2020-08-video]: https://youtu.be/HtKnLiFwHJM
  [rusty-days-2020-08-slides]: https://docs.google.com/presentation/d/1zrxJs7fJgQ29bKfnAll1bYTo9cYZxsCZUwDDtyp5Fak/edit?usp=sharing
  +[production-logging-2020]: https://medium.com/better-programming/production-grade-logging-in-rust-applications-2c7fffd108a6
  Help us expand this list! If you've written or spoken about Tracing, or
  know of resources that aren't listed, please open a pull request adding them.
  