docs: add CHANGELOG.unreleased.md (#5775)

Ref: https://github.com/prettier/prettier/issues/5735#issuecomment-452784056

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,6 +5,7 @@
 
 - [ ] I’ve added tests to confirm my change works.
 - [ ] (If changing the API or CLI) I’ve documented the changes I’ve made (in the `docs/` directory)
+- [ ] (If not an internal change) I’ve added my changes to the `CHANGELOG.unreleased.md` file following the template.
 - [ ] I’ve read the [contributing guidelines](https://github.com/prettier/prettier/blob/master/CONTRIBUTING.md).
 
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CHANGELOG.unreleased.md

@@ -0,0 +1,43 @@
+<!--
+
+Format:
+
+- Category: Title ([#PR] by [@user])
+
+  Description
+
+  ```
+  // Input
+  Code Sample
+
+  // Output (Prettier stable)
+  Code Sample
+
+  // Output (Prettier master)
+  Code Sample
+  ```
+
+Details:
+
+  Description: optional if the `Title` is enough to explain everything.
+
+Examples:
+
+- TypeScript: Correctly handle `//` in TSX ([#5728] by [@JamesHenry])
+
+  Previously, putting `//` as a child of a JSX element in TypeScript led to an error
+  because it was interpreted as a comment. Prettier master fixes this issue.
+
+  <!-- prettier-ignore --\>
+  ```js
+  // Input
+  const link = <a href="example.com">http://example.com</a>
+
+  // Output (Prettier stable)
+  // Error: Comment location overlaps with node location
+
+  // Output (Prettier master)
+  const link = <a href="example.com">http://example.com</a>;
+  ```
+
+-->