docs(no-useless-children-snippet): added rule documentation

Author: marekdedic

docs/rules/no-useless-children-snippet.md

@@ -0,0 +1,57 @@
+---
+pageClass: 'rule-details'
+sidebarDepth: 0
+title: 'svelte/no-useless-children-snippet'
+description: "disallow explicit children snippet where it's not needed"
+---
+
+# svelte/no-useless-children-snippet
+
+> disallow explicit children snippet where it's not needed
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+
+## :book: Rule Details
+
+Any content inside component tags that is not a snippet declaration implicitly becomes part of the children snippet. Thus, declaring the children snippet explicitly is only necessary when the snippet has parameters.
+
+<!--eslint-skip-->
+
+```svelte
+<script>
+  /* eslint svelte/no-useless-children-snippet: "error" */
+
+  import { Foo } from './Foo.svelte';
+</script>
+
+<!-- ✓ GOOD -->
+<Foo>
+  {#snippet bar()}
+    Hello
+  {/snippet}
+</Foo>
+
+<Foo>
+  {#snippet children(val)}
+    Hello {val}
+  {/snippet}
+</Foo>
+
+<Foo>Hello</Foo>
+
+<!-- ✗ BAD -->
+<Foo>
+  {#snippet children()}
+    Hello
+  {/snippet}
+</Foo>
+```
+
+## :wrench: Options
+
+Nothing.
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/src/rules/no-useless-children-snippet.ts)
+- [Test source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/tests/src/rules/no-useless-children-snippet.ts)