Set up default owners and the pull request checklist (#270)

Let's enable the `CODEOWNERS` file so that we do not have to ask for
review manually for each pull request. Also add the pull request
checklist so that we don't forget about the important steps (like
including the release note which was missing in all recent pulls).

.github/CODEOWNERS

@@ -0,0 +1,6 @@
+# This is a comment.
+# Each line is a file pattern followed by one or more owners.
+
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence
+*   @psss @lukaszachy @happz @thrix @janhavlin @martinhoyer

.github/pull_request_template.md

@@ -0,0 +1,7 @@
+## Checklist
+
+* [ ] implement the feature
+* [ ] write the documentation
+* [ ] extend the test coverage
+* [ ] mention the version
+* [ ] include a release note