add 'Write embedded Rust' section

this is for people that have read the book and are looking for crates and
examples. In the future the showcase will be linked from this section.

Author: japaric

index.md

@@ -42,6 +42,15 @@ correct embedded software.
 
 [book]: book
 
+# Write embedded Rust
+
+Once you have learned the basics these resources will make writing embedded
+programs easier.
+
+- [Awesome embedded Rust], a curated list of embedded and no-std crates.
+
+[Awesome embedded Rust]: https://github.com/rust-embedded/awesome-embedded-rust
+
 # Master embedded Rust
 
 Once you're quite familiar with embedded Rust development, you may find these