Added in the `CONTRIBUTING.md` file so that Metacpan's `How to Contribute` link will work.

CONTRIBUTING.md

+# Contributing to the DBD::Mock module
+
+If you'd like to contribute to the DBD::Mock module then I suggest you
+use one of the following ways:
+
+* Post a ticket to the [RT](https://rt.cpan.org/Public/Dist/Display.html?Name=DBD-Mock)
+queue for the module.
+
+* Raise an issue in the [GitLab project](https://gitlab.com/scrapheap/DBD-Mock/issues)
+
+* If you have actual code to commit then make a Merge Request to the [GitLab project](https://gitlab.com/scrapheap/DBD-Mock)