docs: contributing file

CONTRIBUTING.md

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+# Contributing to TermSVG
+
+## Reporting bugs
+
+Open an issue in GitHub issue tracker.
+
+Tell me what's the problem and include steps to reproduce it (reliably).
+Including your OS/browser/terminal name and version in the report would be
+great.
+
+## Submitting patches with bug fixes
+
+If you found a bug and made a patch for it:
+
+1. Make sure your changes pass the [pre-commit](https://pre-commit.com/)
+   [hooks](.pre-commit-config.yaml). You can install the hooks in your work
+   tree by running `pre-commit install` in your checked out copy.
+1. Make sure all tests pass. If you add new functionality, add new tests.
+1. Send a pull request, including a description of the fix (referencing an
+   existing issue if there's one).
+
+## Requesting new features
+
+I'm open to ideas.
+
+If you believe most termsvg users would benefit from implementing your idea
+then feel free to open a GitHub issue. However, as this is an open-source
+project maintained by just one person I simply can't implement all
+of them due to limited resources. Please keep that in mind.
+
+## Proposing features/changes (pull requests)
+
+If you want to propose code change, either introducing a new feature or
+improving an existing one, please first discuss it first. You
+can simply open a separate issue for a discussion.
+
+## Asking for help
+
+GitHub issue tracker is not a support forum but I'll do my best.