add docs

Author: Rich-Harris

site/content/docs/02-template-syntax.md

@@ -111,6 +111,27 @@ Text can also contain JavaScript expressions:
 ```
 
 
+### Comments
+
+---
+
+You can use HTML comments inside components.
+
+```html
+<!-- this is a comment! -->
+<h1>Hello world</h1>
+```
+
+---
+
+Comments beginning with `svelte-ignore` disable warnings for the next block of markup. Usually these are accessibility warnings; make sure that you're disabling them for a good reason.
+
+```html
+<!-- svelte-ignore a11y-autofocus -->
+<input bind:value={name} autofocus>
+```
+
+
 ### {#if ...}
 
 ```sv