docs: add build, packaging and development instructions

CONTRIBUTING.md

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+# Contributing / Development
+Instructions for development setup and upstreaming patches.
+
+If you just want to build or package quickshell see [BUILD.md](BUILD.md).
+
+## Development
+Install the dependencies listed in [BUILD.md](BUILD.md).
+You probably want all of them even if you don't use all of them
+to ensure tests work correctly and avoid passing a bunch of configure
+flags when you need to wipe the build directory.
+
+Quickshell also uses `just` for common development command aliases.
+
+The dependencies are also available as a nix shell or nix flake which we recommend
+using with nix-direnv.
+
+Common aliases:
+- `just configure [<debug|release> [extra cmake args]]` (note that you must specify debug/release to specify extra args)
+- `just build` - runs the build, configuring if not configured already.
+- `just run [args]` - runs quickshell with the given arguments
+- `just clean` - clean up build artifacts. `just clean build` is somewhat common.
+
+### Formatting
+All contributions should be formatted similarly to what already exists.
+Group related functionality together.
+
+Run the formatter using `just fmt`.
+If the results look stupid, fix the clang-format file if possible,
+or disable clang-format in the affected area
+using `// clang-format off` and `// clang-format on`.
+
+### Linter
+All contributions should pass the linter.
+
+Note that running the linter requires disabling precompiled
+headers and including the test codepaths:
+```sh
+$ just configure debug -DNO_PCH=ON -DBUILD_TESTING=ON
+$ just lint
+```
+
+If the linter is complaining about something that you think it should not,
+please disable the lint in your MR and explain your reasoning.
+
+### Tests
+If you feel like the feature you are working on is very complex or likely to break,
+please write some tests. We will ask you to directly if you send in an MR for an
+overly complex or breakable feature.
+
+At least all tests that passed before your changes should still be passing
+by the time your contribution is ready.
+
+You can run the tests using `just test` but you must enable them first
+using `-DBUILD_TESTING=ON`.
+
+### Documentation
+Most of quickshell's documentation is automatically generated from the source code.
+You should annotate `Q_PROPERTY`s and `Q_INVOKABLE`s with doc comments. Note that the parser
+cannot handle random line breaks and will usually require you to disable clang-format if the
+lines are too long.
+
+Before submitting an MR, if adding new features please make sure the documentation is generated
+reasonably using the `quickshell-docs` repo.
+
+Doc comments take the form `///` or `///!` (summary) and work with markdown.
+Look at existing code for how it works.
+
+Quickshell modules additionally have a `module.md` file which contains a summary, description,
+and list of headers to scan for documentation.