Book: Add continuous integration description

Author: flip1995

book/src/SUMMARY.md

@@ -7,8 +7,8 @@
 - [Configuration](configuration.md)
 - [Clippy's Lints](lints.md)
 - [Continuous Integration](continuous_integration/README.md)
-    - [Travis CI](continuous_integration/travis.md)
     - [GitHub Actions](continuous_integration/github_actions.md)
+    - [Travis CI](continuous_integration/travis.md)
 - [Development](development/README.md)
     - [Basics](development/basics.md)
     - [Adding Lints](development/adding_lints.md)

book/src/continuous_integration/README.md

@@ -1 +1,17 @@
 # Continuous Integration
+
+It is recommended to run Clippy on CI, preferably with `-Dwarnings`, so that
+Clippy lints prevent CI from passing.
+
+We recommend to use Clippy from the same toolchain, that you use for compiling
+your crate for maximum compatibility. E.g. if your crate is compiled with the
+`stable` toolchain, you should also use `stable` Clippy.
+
+> _Note:_ New Clippy lints are first added to the `nightly` toolchain. If you
+> want to help with improving Clippy and have CI resources left, please consider
+> adding a `nightly` Clippy check to your CI and report problems like false
+> positives back to us. With that we can fix bugs early, before they can get to
+> stable.
+
+This chapter will give an overview on how to use Clippy on different popular CI
+providers.