.github/CODEOWNERS: Added the CODEOWNERS file

This should make it safe to start playing with protected branch settings.
diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
new file mode 100644
index 0000000..18bacfc
--- /dev/null
+++ b/.github/CODEOWNERS
@@ -0,0 +1,6 @@
+# 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.
+#
+*       @gtristan @juergbi @BenjaminSchubert @cs-shadow