Add why paragraph to the README.md

Author: svartalf

README.md

@@ -12,6 +12,7 @@ This GitHub Action provides faster version of the `cargo install` command.
 
 **Table of Contents**
 
+* [Why?](#why)
 * [How does this works?](#how-does-it-work)
 * [Example workflow](#example-workflow)
 * [Inputs](#inputs)
@@ -21,6 +22,12 @@ This GitHub Action provides faster version of the `cargo install` command.
 * [License](#license)
 * [Contribute and support](#contribute-and-support)
 
+## Why?
+
+If you are using binary crates (such as [`cargo-audit`](https://crates.io/crates/cargo-audit), [`grcov`](https://github.com/mozilla/grcov), [`cargo-geiger`](https://crates.io/crates/cargo-geiger) and so on) in your CI workflows, you might have noticed that compiling these crates each time is irritatingly slow.
+
+This Action speeds up the crates installation with some tricks, leading to a much faster job execution; crates are expected to be installed in a couple seconds.
+
 ## How does it work?
 
 Before calling your usual `cargo install` command, this Action