docs: add docs for #[related]

README.md

@@ -41,6 +41,7 @@ and such might not want.
   - [... in `main()`](#-in-main)
   - [... diagnostic code URLs](#-diagnostic-code-urls)
   - [... snippets](#-snippets)
+  - [... multiple related errors](#-multiple-related-errors)
   - [... handler options](#-handler-options)
 - [Acknowledgements](#acknowledgements)
 - [License](#license)
@@ -354,6 +355,26 @@ pub struct MyErrorType {
 }
 ```
 
+### ... multiple related errors
+
+`miette` supports collecting multiple errors into a single diagnostic, and
+printing them all together nicely.
+
+To do so, use the `#[related]` tag on any `IntoIter` field in your
+`Diagnostic` type:
+
+```rust
+use miette::Diagnostic;
+use thiserror::Error;
+
+#[derive(Debug, Error, Diagnostic)]
+#[error("oops")]
+struct MyError {
+    #[related]
+    Vec<MyError>,
+}
+```
+
 ### ... handler options
 
 [MietteHandler] is the default handler, and is very customizable. In most