Explain what each dependency is for

README.md

@@ -20,22 +20,30 @@ You may be looking for:
 
 ## Serde in action
 
-**Cargo.toml**:
+<details>
+<summary>
+Click to show Cargo.toml.
+<a href="http://play.integer32.com/?gist=9003c5b88c1f4989941925d7190c6eec" target="_blank">Run this code in the playground.</a>
+</summary>
 
 ```toml
 [dependencies]
+
+# The core APIs, including the Serialize and Deserialize traits. Always
+# required when using Serde.
 serde = "1.0"
+
+# Support for #[derive(Serialize, Deserialize)]. Required if you want Serde
+# to work for structs and enums defined in your crate.
 serde_derive = "1.0"
+
+# Each data format lives in its own crate; the sample code below uses JSON
+# but you may be using a different one.
 serde_json = "1.0"
 ```
 
-Click on the "Run" button to have a quick taste:
-
-<a href="http://play.integer32.com/?gist=9003c5b88c1f4989941925d7190c6eec" target="_blank">
-<img align="right" width="50" src="https://raw.githubusercontent.com/serde-rs/serde-rs.github.io/master/img/run.png">
-</a>
-
-**src/lib.rs**:
+</details>
+<p></p>
 
 ```rust
 #[macro_use]