Add README.md with some basic information

README.md

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+# curlnoise
+
+A [Quil](http://www.quil.info/) sketch which implements what is
+described in [Curl-Noise for Procedural Fluid
+Flow](https://www.cs.ubc.ca/~rbridson/docs/bridson-siggraph2007-curlnoise.pdf).
+
+Very much a work-in-progress as I am not especially experienced in
+Clojure, nor in the mathematics of this paper.
+
+## Usage
+
+This runs in both Clojure via `lein run`, and ClojureScript via `lein
+compile` or `lein figwheel`.
+
+Uncomment the `(run-sketch)` and the below should work:
+
+- LightTable - open `core.clj` and press `Ctrl+Shift+Enter` to evaluate the file.
+- Emacs - run cider, open `core.clj` and press `C-c C-k` to evaluate the file.
+- REPL - run `(require 'quil_perlin.core)`.
+
+## TODO
+
+- Better docs.
+- Make things more interactive, e.g. let the user place obstacles.
+- Factor out the potential function rather than burying it in
+  `update-state`.
+- Visualize amplitude function & potential function.  I have
+  `draw-field` which `show-fn` enables, but it is rudimentary.
+- Figure out reasonable boundary behavior.  I'd like to do toroidal,
+  but I would need to gradually blend the potential function at the
+  edges for this to work right (otherwise particles are just stuck at
+  the boundaries due to the discontinuity).
+- Use 'real' simplex or OpenSimplex noise implementation that has a
+  gradient instead of doing it numerically?