project: add optional To do list to PR template

https://git.minetest.land/Mineclonia/Mineclonia/pulls/56#issuecomment-22465

.gitea/pull_request_template.md

@@ -25,3 +25,19 @@ don't need them all, delete the empty numbers.
 1. 
 2. 
 3. 
+
+<!--
+If your pull request still needs work, uncomment the
+following section and include a list of things that
+need to be done before it's ready for us to look at.
+
+Please remember to put WIP: in front of the title as well.
+-->
+
+<!--
+## To do
+
+- [ ] Item 1
+- [ ] Item 2
+- [ ] Item 3
+-->