[vcpkg] Add pull request template (#9280)

* [vcpkg] Add pull request template

* Update pull_request_template.md

.github/pull_request_template.md

@@ -0,0 +1,7 @@
+**Describe the pull request**
+
+- What does your PR fix? Fixes issue #
+
+- Which triplets are supported/not supported? Have you updated the CI baseline?
+
+- Does your PR follow the [maintainer guide](https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/maintainer-guide.md)?