Add PULL_REQUEST_TEMPLATE

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+# PR Prelude
+
+Thank you for working on SpaceVim! :)
+
+**Please complete these steps and check these boxes (by putting an `x` inside
+the brackets) _before_ filing your PR:**
+
+- [ ] I have read and understood SpaceVim's [CONTRIBUTING][cont] document.
+- [ ] I have read and understood SpaceVim's [CODE_OF_CONDUCT][code] document.
+- [ ] I have included tests for the changes in my PR. If not, I have included a
+  rationale for why I haven't.
+- [ ] **I understand my PR may be closed if it becomes obvious I didn't
+  actually perform all of these steps.**
+
+# Why this change is necessary and useful
+
+[Please explain **in detail** why the changes in this PR are needed.]
+
+[cont]: https://github.com/SpaceVim/SpaceVim/blob/dev/CONTRIBUTING.md
+[code]: https://github.com/SpaceVim/SpaceVim/blob/dev/CODE_OF_CONDUCT.md