Add project contribution standards

CONTRIBUTING.md

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+# Contributing to Herostratus
+
+## Project goals
+
+This is a silly project to gamify things that shouldn't be gamified.
+
+* The achievements should be whimsical and silly, and absolutely not be used in a serious manner,
+  especially not to measure developer contribution.
+* It should be easyish to set up by a new user
+    * Its target audience is software engineers, so it need not be too easy
+* Its target runtime environment is modern GNU Linux x86_64
+    * It could also support other environments, but they won't be prioritized
+* It should be easyish to add new rules
+* It should be fast
+* Its preferred UI is the CLI
+    * It could provide a standalone web UI
+* It should provide optional integrations:
+    * It should provide an integration with GitLab achievements
+    * It could provide an integration with GitHub
+
+## Tests
+
+This project values tests. Please consider adding tests with contributions. That said, I do not
+believe in 100% test coverage (or even chasing after a certain % number). Please use the following
+principles:
+
+* Tests should be preferred in the following priority: Unit, Integration, Manual.
+    * This repository does contain a few orphan test branches prefixed with `test/`. These branches
+      can be used for integration tests, but please ensure that the repository size does not
+      explode.
+* Okay tests are better than no tests. Bad tests are worse than no tests.
+* Unit test the things that can provide value when tested
+* Don't make code more complex just to make it testable
+
+## Build warnings
+
+Compiler warnings are treated as errors in the CI/CD pipeline. Similarly, the CI/CD pipeline runs
+Clippy, and treats all lints as errors. Lints may be judiciously ignored on a case-by-case basis.
+
+However, warnings are a normal part of the development workflow, and are thus left enabled for local
+developers.
+
+Outdated dependencies should be updated.
+
+New lints added by the latest stable Rust toolchain should be resolved (but in atomic commits!).
+
+## MSRV
+
+Herostratus is a CLI application, and thus there is no need to maintain compatibility with older
+Rust toolchains, or older versions of dependencies.
+
+The Minimum Supported Rust Version (MSRV) is the latest stable toolchain.
+
+## Logging
+
+Logs are great.
+
+* INFO level logs should not be spammy
+* Prefer logs that provide information. E.g., instead of `"Failed to process repository"`, indicate
+  _what_ and _why_ with `"failed to process repository '{repo}' because: '{e:?}'"`
+
+## Git
+
+This repository uses a "sawtooth" merge strategy. All PRs are to be rebased on top of `main` prior
+to merging. Single commit PRs may be merged with a fast-forward merge. Multi-commit PRs shall be
+merged with a merge commit.
+
+**This project values its Git history.**
+
+"Squash and merge" PRs will not be accepted, and neither will PRs using "Conventional Commits" to an
+excessive degree. I may choose to accept such PRs, but rewrite the commits myself before merging. I
+will do my best to maintain original commit authorship.
+
+Subjective judgment calls will be made by me (@Notgnoshi). Heated arguments about what is "right"
+will be ignored or shut down, but the I _am_ willing to work with PR authors who don't normally
+develop according to the my preferences.
+
+**The matter of Git history is a subjective one, with no universally correct practices.**
+
+However, see <https://cbea.ms/git-commit/> and
+<https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html> for the set of guiding
+principles that this project uses for its Git history.
+
+Unfortunately, the principles that make for the best commit messages aren't possible to lint
+automatically. So instead, this project uses `gitlint` in the CI/CD pipeline to lint the less
+important, but _possible_ to lint mechanics.
+
+You can run `gitlint` yourself with the same rules as the pipeline by setting the following two
+environment variables
+```sh
+# Run from within the Herostratus repository directory
+export GITLINT_CONFIG="$PWD/.github/gitlint/gitlint.ini"
+export GITLINT_EXTRA_PATH="$PWD/.github/gitlint"
+```
+You can invoke gitlint through the CLI, your editor, or as a commit hook.
+
+In my experience, enforcing these rules is enough to communicate that a project cares about its
+commit history, which results in an overall better history. And I've found that projects that
+maintain their history are _far_ easier to do code archaeology on (which is a personal value of
+mine).
+
+**I don't desire a perfect history. Just a good one.**