[contributing] Add issue template for exercise discussion

Author: woojiahao

.github/ISSUE_TEMPLATE/exercise_discussion.md

@@ -0,0 +1,26 @@
+---
+name: Exercise Discussions
+about: Want to create a new exercise for Git-Mastery? Discuss the details of the exercise with the Git-Mastery team first.
+title: "[Exercise Discussion]"
+labels: ["exercise-discussion"]
+assignees: 
+  - woojiahao
+  - damithc
+---
+
+# Exercise Discussion
+
+## 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 verified that this exercise does not already exist or is not currently in review?