Auto merge of rust-lang#9944 - flip1995:move-syntax-tree-patterns, r=Manishearth

Move syntax tree patterns RFC to the book

r? `@Manishearth`

Follow up to rust-lang#3875

changelog: none

Author: bors

book/src/SUMMARY.md

@@ -21,3 +21,4 @@
         - [The Clippy Book](development/infrastructure/book.md)
     - [Proposals](development/proposals/README.md)
         - [Roadmap 2021](development/proposals/roadmap-2021.md)
+        - [Syntax Tree Patterns](development/proposals/syntax-tree-patterns.md)