git: Add codeowners file to automatically request code reviews

.github/CODEOWNERS

@@ -0,0 +1,14 @@
+# Code owners are automatically requested for review when someone
+# opens a pull request that modifies code that they own. When someone
+# with admin or owner permissions has enabled required reviews, they
+# also can optionally require approval from a code owner before the
+# author can merge a pull request in the repository.
+
+wallet/                 @cdecker
+*.py                    @cdecker
+
+wallet/invoices.*       @ZmnSCPxj
+lightningd/payalgo.*    @ZmnSCPxj
+
+# See https://help.github.com/articles/about-codeowners/ for more
+# information