Update posts/2021-10-21-Rust-1.56.0.md

Co-authored-by: Mark Rousskov <[email protected]>

Author: pietroalbini

posts/2021-10-21-Rust-1.56.0.md

@@ -25,7 +25,8 @@ from the appropriate page on our website, and check out the
 
 ### Rust 2021
 
-We wrote about plans for Rust 2021 [in May](https://blog.rust-lang.org/2021/05/11/edition-2021.html).
+We wrote about plans for the Rust 2021 Edition [in May](https://blog.rust-lang.org/2021/05/11/edition-2021.html).
+Editions are a mechanism for opt-in changes that may otherwise pose backwards compatibility risk. See [the edition guide](https://doc.rust-lang.org/stable/edition-guide/editions/index.html) for details on how this is achieved.
 This a smaller edition, especially compared to 2018, but there
 are still some nice quality-of-life changes that require an edition opt-in to
 avoid breaking some corner cases in existing code. See the new chapters of the