feat: add border with top triangle snippet

snippets/border-with-top-triangle.md

@@ -0,0 +1,60 @@
+### Border with top triangle
+
+Use pure CSS to create div with top triangle.
+
+#### HTML
+
+```html
+<div class="container">
+  Border with top triangle
+</div>
+```
+
+#### CSS
+
+```css
+.container {
+  display: block;
+  position: relative;
+  background: #ffffff;
+  padding: 15px;
+  border: 1px solid #dddddd;
+  margin-top: 20px;
+}
+
+.container:before, .container:after {
+  content: '';
+  display: block;
+  position: absolute;
+  bottom: 100%;
+  width: 0;
+  height: 0;
+}
+
+.container:before {
+  left: 19px;
+  border: 11px solid transparent;
+  border-bottom-color: #dddddd;
+}
+
+.container:after {
+  left: 20px;
+  border: 10px solid transparent;
+  border-bottom-color: #ffffff;
+}
+```
+
+#### Demo
+
+#### Explanation
+
+1. Use pseudo-element `before` and `after` to create two triangles. How to create triangle can see the [Triangle](https://30-seconds.github.io/30-seconds-of-css/#triangle). 
+2. The color of the `before` triangle as same as the container border color. The color of the `after` triangle as same as the container background color.
+3. The border width of the `before` triangle is wider 1px(depend on the border width of container) than the `after` triangle, then we can see the top triangle border on the container.
+4. The `after` triangle is on the right of the `before` triangle(1px) so that can make the left part border of the `before` triangle to visible.
+
+#### Browser support
+
+<span class="snippet__support-note">✅ No caveats.</span>
+
+<!-- tags: visual -->