Document cargo-clippy feature

It is possible to use conditional compilation to prevent Clippy from
evaluating certain code at all. Unfortunately, it was no longer
documented anywhere. This adds a brief explanation of how to use the
feature with conditional compilation, and mentions a few downsides.

Fixes #rust-lang#10220 — Ability to skip files or blocks entirely

book/src/configuration.md

@@ -73,6 +73,27 @@ interested in:
 cargo clippy -- -A clippy::all -W clippy::useless_format -W clippy::...
 ```
 
+### Disabling evaluation of certain code
+
+Rarely, you may wish to prevent Clippy from evaluating certain sections of code entirely. Clippy sets the `cargo-clippy`
+feature when it compiles your code, so you can use [conditional
+compilation](https://doc.rust-lang.org/reference/conditional-compilation.html) to prevent files or blocks from being
+parsed at all. Note that since the code is not evaluated you cannot turn evaluation or lints back on within the code.
+Also, if the code is referenced from elsewhere in your crate, you may need to provide a stub so that the code compiles:
+
+```rust
+#[cfg(not(feature = "cargo-clippy"))]
+include!(concat!(env!("OUT_DIR"), "/my_big_function-generated.rs"));
+
+#[cfg(feature = "cargo-clippy")]
+fn my_big_function(_input: &str) -> Option<MyStruct> {
+    None
+}
+```
+
+This feature is not actually part of your crate, so specifying `--all-features` to other tools, e.g. `cargo test
+--all-features`, will not disable it.
+
 ### Specifying the minimum supported Rust version
 
 Projects that intend to support old versions of Rust can disable lints pertaining to newer features by specifying the