Create the readme.

README.md

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+# driftingnebula
+
+> Generative art with GIMP, GEGL and ImageMagick.
+
+## Usage
+
+To run the generations you'll need to have [GEGL 0.4], [git], [ImageMagick 7], [NodeJS] and [pnpm] installed. Then in a terminal, you can do the following things.
+
+[GEGL 0.4]: https://gegl.org
+[git]: https://git-scm.com
+[ImageMagick 7]: https://imagemagick.org/
+[NodeJS]: https://nodejs.org
+[pnpm]: https://pnpm.io
+
+```sh
+# Download the repository.
+git clone https://github.com/driftingnebula/driftingnebula
+cd driftingnebula
+
+# Install the NodeJS dependencies.
+pnpm install
+
+# Run the generations. See the "output" directory for files.
+pnpm start
+
+# Run the generations but don't render anything.
+pnpm start -- --no-render
+
+# Run the generations and explicitly include all the default
+# parameters for GEGL operations.
+pnpm start -- --include-defaults
+
+# Lint the code and run tests.
+pnpm test
+```