GH: add PR template (#6482)

.github/PULL_REQUEST_TEMPLATE.md

+Thank you for submitting a YOLOv5 🚀 Pull Request! We want to make contributing to YOLOv5 as easy and transparent as possible. A few tips to get you started:
+
+- Search existing YOLOv5 [PRs](https://github.com/ultralytics/yolov5/pull) to see if a similar PR already exists.
+- Link this PR to a YOLOv5 [issue](https://github.com/ultralytics/yolov5/issues) to help us understand what bug fix or feature is being implemented.
+- Provide before and after profiling/inference/training results to help us quantify the improvement your PR provides (if applicable).
+
+Please see our ✅ [Contributing Guide](https://github.com/ultralytics/yolov5/blob/master/CONTRIBUTING.md) for more details.

CONTRIBUTING.md

@@ -42,17 +42,17 @@ changes** button. All done, your PR is now submitted to YOLOv5 for review and ap
 To allow your work to be integrated as seamlessly as possible, we advise you to:
 
 - ✅ Verify your PR is **up-to-date with upstream/master.** If your PR is behind upstream/master an
-  automatic [GitHub actions](https://github.com/ultralytics/yolov5/blob/master/.github/workflows/rebase.yml) rebase may
-  be attempted by including the /rebase command in a comment body, or by running the following code, replacing 'feature'
-  with the name of your local branch:
-
-  ```bash
-  git remote add upstream https://github.com/ultralytics/yolov5.git
-  git fetch upstream
-  git checkout feature  # <----- replace 'feature' with local branch name
-  git merge upstream/master
-  git push -u origin -f
-  ```
+  automatic [GitHub Actions](https://github.com/ultralytics/yolov5/blob/master/.github/workflows/rebase.yml) merge may
+  be attempted by writing /rebase in a new comment, or by running the following code, replacing 'feature' with the name
+  of your local branch:
+
+```bash
+git remote add upstream https://github.com/ultralytics/yolov5.git
+git fetch upstream
+# git checkout feature  # <--- replace 'feature' with local branch name
+git merge upstream/master
+git push -u origin -f
+```
 
 - ✅ Verify all Continuous Integration (CI) **checks are passing**.
 - ✅ Reduce changes to the absolute **minimum** required for your bug fix or feature addition. _"It is not daily increase