Add The Little Book of Rust Macros (#2911)

I found this recently when reading about hygiene in Rust macros and
really liked it.

Author: mgeisler

src/other-resources.md

@@ -63,6 +63,8 @@ A small selection of other guides and tutorial for Rust:
   Lists](https://rust-unofficial.github.io/too-many-lists/): in-depth
   exploration of Rust's memory management rules, through implementing a few
   different types of list structures.
+- [The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/): covers
+  many details on Rust macros with practical examples.
 
 Please see the [Little Book of Rust Books](https://lborb.github.io/book/) for
 even more Rust books.