[contributing] Add PR template

Author: woojiahao

.github/PULL_REQUEST_TEMPLATE/submit_exercise.md

@@ -0,0 +1,28 @@
+---
+name: Submit Exercise for Review
+about: Creating a new exercise for Git-Mastery? Submit it through exercise review.
+title: [Exercise Review]
+labels: exercise-review
+assignees: woojiahao
+---
+
+# Exercise Review
+
+## Exercise Objectives
+
+- What are the objectives of this exercise?
+- Which tour and lesson does this exercise belong to?
+- What are the core concepts tested in this exercise?
+
+## Exercise Grading Setup
+
+- How is the exercise setup for the exercise?
+- What is the grading scheme for the exercise?
+
+## Checklist
+
+- [ ] Have you written unit tests using [`repo-smith`](https://github.com/git-mastery/repo-smith) to validate the exercise grading scheme?
+- [ ] Have you tested the download script using `test-download.sh`?
+- [ ] Have you verified that this exercise does not already exist or is not currently in review?
+- [ ] Did you introduce a new grading mechanism that should belong to [`git-autograder`](https://github.com/git-mastery/git-autograder)?
+- [ ] Did you introduce a new dependency that should belong to [`app`](https://github.com/git-mastery/app)?