Add a template for merge requests

.gitlab/merge_request_templates/default.md

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+You're opening a merge request to BondageProject's BondageClub project, and we have a couple prerequisites:
+
+- its title will end up in the changelog for the release
+
+If you're adding assets, something like `Adds a new thing, a glorb and a foo` would be enough.
+If you have additional credits because it's a combined project, state so in the description.
+
+Please make it as clear, concise, and high-level as possible, given that's most of the readers
+want from the changelog. If you have technical details, those stay in the description.
+
+- please check CI before opening
+
+We have a bot that runs various checks on incoming MRs and catches issues early (which is nice).
+But GitGud has it under a whitelist, so if you're a new contributor it won't get run (which is
+sad). Thankfully, you can run those checks locally yourself using the following steps, provided
+you have access to a shell:
+
+```sh
+# only needs to happen once in a while; this requires Node to be installed
+npm install
+# runs the same script the CI bot runs
+npm run checks
+```
+
+This will give you the same message a CI run here will, pointing out at issues with asset
+configuration and whitespace. If you get any questions about what it means, or are struggling to
+run the checks, ask for help in BC's #programming Discord channel.
+
+In case of whitespace errors, you can additionally run
+```sh
+npm run assets:lint:fix scripts:lint:fix
+```
+which will automatically fix most of them for you, so you only need to commit those afterwards.
+
+Thank you for your contribution!
+
+>8 >8 >8 >8 >8 All lines above this can be deleted once read 8< 8< 8< 8< 8<
+
+%{first_multiline_commit}