feat: Add Pull Request template (#464)

Add a PR template to make it easier for contributors to
submit PRs with all the necessary information.

Author: rebornplusplus

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,33 @@
+<!--
+	Point to the github issue number this pull request closes.
+	Example:
+
+		Closes #123.
+-->
+
+Closes **TODO**.
+
+<!--
+	Include the LightOJ Problem url the changes are about.
+	Example:
+
+		Problem link: https://lightoj.com/problem/1000.
+-->
+
+Problem link: **TODO**.
+
+### Changes introduced in this PR
+
+<!--
+	Write a summary of the changes that are included in this PR.
+	Examples:
+
+		- This PR adds an English tutorial for problem 1000.
+		- This PR fixes source code of the previously written
+		  tutorial of 1000.
+		- This PR adds a new solution approach.
+		- ...
+-->
+
+**TODO**
+