mention restriction about nested rules starting with an identifier

plugins/postcss-nesting/README.md

@@ -81,6 +81,36 @@ Future versions of this plugin will first warn and then error if you use `@nest`
 
 We advice everyone to migrate their codebase **now** to nested CSS without `@nest`.
 
+## ⚠️ Nested selectors must start with a symbol
+
+The current version of the [CSS Nesting specification](https://www.w3.org/TR/2023/WD-css-nesting-1-20230214/#example-34e8e94f) disallows nested selectors to start with a letter (i.e. a tag name or element selector). To write such selectors, they need to be prefixed with `& ` or wrapped with `:is()`.
+
+You will get a warning when selectors start with a letter:
+> Nested selectors must start with a symbol and "span" begins with a letter.
+
+```pcss
+.foo {
+	/* ❌ invalid */
+	span {
+		color: hotpink;
+	}
+
+	/* ✅ valid */
+	& span {
+		color: hotpink;
+	}
+
+	/* ❌ invalid */
+	span & {
+		color: hotpink;
+	}
+
+	/* ✅ valid */
+	:is(span) & {
+		color: hotpink;
+	}
+}	
+```
 
 ## Options
 

plugins/postcss-nesting/docs/README.md

@@ -42,6 +42,36 @@ Future versions of this plugin will first warn and then error if you use `@nest`
 
 We advice everyone to migrate their codebase **now** to nested CSS without `@nest`.
 
+## ⚠️ Nested selectors must start with a symbol
+
+The current version of the [CSS Nesting specification](https://www.w3.org/TR/2023/WD-css-nesting-1-20230214/#example-34e8e94f) disallows nested selectors to start with a letter (i.e. a tag name or element selector). To write such selectors, they need to be prefixed with `& ` or wrapped with `:is()`.
+
+You will get a warning when selectors start with a letter:
+> Nested selectors must start with a symbol and "span" begins with a letter.
+
+```pcss
+.foo {
+	/* ❌ invalid */
+	span {
+		color: hotpink;
+	}
+
+	/* ✅ valid */
+	& span {
+		color: hotpink;
+	}
+
+	/* ❌ invalid */
+	span & {
+		color: hotpink;
+	}
+
+	/* ✅ valid */
+	:is(span) & {
+		color: hotpink;
+	}
+}	
+```
 
 ## Options