Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribution guidelines
+
+## Development environment
+
+### Prerequisites
+
+- PHP (version TBD)
+- `make`
+- `entr` to watch for file changes
+- `xdg-open` link handler to invoke browser
+- patience
+
+### Cloning or updating the repository
+
+Ensure the consistency of the `languages` submodule by using the following options:
+
+- `git clone --recurse-submodules <repository-url>`
+- `git pull --recurse-submodules`
+
+### Development
+
+Run at least once: `make fetch` to query servers. This can take around 5 minutes.
+
+Run when developing: `make dev` to watch for changes & serve HTML locally in browser.
+Does not respond to new files.
+
+See [`Makefile`](Makefile) for more details.
+
+### Running your own copy
+
+- point your webserver at the `output` folder
+- install systemd services from the `systemd` folder or an equivalent timer
+- `session_sudoers`: TBD
+
+## Code style guidelines
+
+### General
+
+**Indentation**: Tabs (4-wide)
+
+**Filename seperator**: Hyphen (`-`)
+
+### PHP
+
+**Identifier casing**: `snake_case` and `CONSTANT_CASE`
+
+**Comments and documentation**: TBD
+
+### HTML & CSS
+
+**Identifier casing**: `kebab-case`, occasional `snake_case`
+
+**Comments and documentation**: TBD
+
+### JavaScript
+
+**Identifier casing**: `camelCase` and `CONSTANT_CASE`, occasional `snake_case`
+
+**Comments and documentation**: [JSDoc](https://jsdoc.app/)
+
+## Contact
+
+- Web Development Session Community on [caliban.org](https://sog.caliban.org/)
+- Project lead, querying logic, deployment, community filtering: `someguy` on Session
+- Documentation, code quality, HTML generation, CSS, JS: `gravel` on Session