Add Renovate to mdbook

I'd like to start experimenting with using Renovate in the mdbook
repository.

Author: ehuss

repos/rust-lang/mdBook.toml

@@ -2,7 +2,7 @@ org = "rust-lang"
 name = "mdBook"
 description = "Create book from markdown files. Like Gitbook but implemented in Rust"
 homepage = "https://rust-lang.github.io/mdBook/"
-bots = ["rustbot"]
+bots = ["rustbot", "renovate"]
 
 [access.teams]
 devtools = "write"