Book: Add infrastructure description

Author: flip1995

book/src/development/infrastructure/README.md

@@ -1 +1,19 @@
 # Infrastructure
+
+In order to deploy Clippy over `rustup`, some infrastructure is necessary. This
+chapter describes the different parts of the Clippy infrastructure that need to
+be maintained to make this possible.
+
+The most important part is the sync between the `rust-lang/rust` repository and
+the Clippy repository that takes place every two weeks. This process is
+described in the [Syncing changes between Clippy and `rust-lang/rust`](sync.md)
+section.
+
+A new Clippy release is done together with every Rust release, so every six
+weeks. The release process is described in the [Release a new Clippy
+Version](release.md) section. During a release cycle a changelog entry for the
+next release has to be written. The format of that and how to do that is
+documented in the [Changelog Update](changelog_update.md) section.
+
+> _Note:_ The Clippy CI should also be described in this chapter, but for now is
+> left as a TODO.