Add a CONTRIBUTING.md file

Author: Leont

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Contributing to perl
+
+There are several documents trying to help you on your way to contribute to our project,
+including F<pod/perlhack.pod>, F<pod/perlhacktips.pod> and C<pod/perlhacktut.pod>.
+
+Please be mindful of `CODE_OF_CONDUCT.md` and `AI_POLICY.md`.
+
+## In brief:
+
+ * Before opening any non-trivial pull request, open an issue first. Starting to code before there's agreement on how to approach something often leads to wasted effort.
+
+ * Be sure run C<make test» before opening a PR, or C<TEST_JOBS=$(nproc) make -j$(nproc) test_harness> if you're in a hurry.
+
+ * If this is your first contribution, please don't forget to run `perl Porting/updateAUTHORS.pl` after committing (and committing the result), otherwise the F<AUTHOR> file tests will start failing.