Add attribute documentation

Author: nyurik

book/src/SUMMARY.md

@@ -4,6 +4,7 @@
 
 - [Installation](installation.md)
 - [Usage](usage.md)
+    - [Extending Clippy coverage](lib_usage.md)
 - [Configuration](configuration.md)
     - [Lint Configuration](lint_configuration.md)
 - [Clippy's Lints](lints.md)

book/src/lib_usage.md

@@ -0,0 +1,22 @@
+# Modifying Clippy behavior with attributes
+
+In some cases it is possible extend Clippy coverage to include 3rd party libraries.  At this moment, only one such modification is possible: adding a `#[clippy::format_args]` attribute to a macro that supports `format!`-like syntax.
+
+## `#[clippy::format_args]`
+
+This attribute can be added to a macro that supports `format!`-like syntax.  It tells Clippy that the macro is a formatting macro, and that the arguments to the macro should be linted as if they were arguments to `format!`.  Any lint that would apply to a `format!` call will also apply to the macro call. The macro may have additional arguments before the format string, and these will be ignored.
+
+### Example
+
+```rust
+/// A macro that prints a message if a condition is true
+#[macro_export]
+#[clippy::format_args]
+macro_rules! print_if {
+    ($condition:expr, $($args:tt)+) => {{
+        if $condition {
+            println!($($args)+)
+        }
+    }};
+}
+```