Add a comment reminding to update README if the default changes

Author: sigustin

clippy_lints/src/lib.rs

@@ -46,6 +46,8 @@ extern crate syntax_pos;
 
 use toml;
 
+// Currently, categories "style", "correctness", "complexity" and "perf" are enabled by default,
+// as said in the README.md of this repository. If this changes, please update README.md.
 macro_rules! declare_clippy_lint {
     { pub $name:tt, style, $description:tt } => {
         declare_tool_lint! { pub clippy::$name, Warn, $description, report_in_external_macro: true }