Add CONTRIBUTING.md

Author: robertoffmoura

CONTRIBUTING.md

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+# Contributing
+
+Thanks for helping out! This converter runs in both MATLAB and GNU Octave, so
+changes should work in both. Below is how to run the tests and how to add
+support for a new plot type or gallery example.
+
+## Running the tests
+
+The test runner is `plotly/testing/runplotlytests.m`; it discovers every
+`Test_*.m` under `plotly/`.
+
+```matlab
+addpath(fullfile(pwd, 'plotly', 'testing'));
+runplotlytests                                              % everything
+runplotlytests('Test_m2json')                               % one suite
+runplotlytests('Test_plotlyfig_lines/testLinePlotData')     % one test
+```
+
+In Octave, install the optional packages once if you haven't:
+`pkg install -forge datatypes statistics`.
+
+## Adding a plot handler
+
+Every supported graphics type has an `updateX` function in
+`plotly/plotlyfig_aux/handlegraphics/`. To add one:
+
+1. Inspect the object first. Octave and MATLAB expose different graphics
+   objects. Use `get`/`set` rather than dot indexing.
+2. Add `plotly/plotlyfig_aux/handlegraphics/updateX.m`:
+
+   ```matlab
+   function obj = updateX(obj, dataIndex)
+       h = obj.State.Plot(dataIndex).Handle;
+       obj.data{dataIndex}.type = 'scatter';
+       obj.data{dataIndex}.x = get(h, 'XData');
+       obj.data{dataIndex}.y = get(h, 'YData');
+   end
+   ```
+3. Register it in the switch in `plotly/plotlyfig_aux/core/updateData.m`.
+4. Add a test to the matching suite (`Test_plotlyfig_lines`, `_bars`, `_3d`
+   or `_special`).
+
+Reusable code belongs in `plotly/plotlyfig_aux/helpers/`; local functions in
+one file are not visible from another.
+
+## Adding a gallery entry
+
+The gallery compares native and converted figures for each entry in
+`galleryEntries()` at the bottom of `plotly/makegallery.m`. Add a `{name, code}`
+row:
+
+```matlab
+'myplot',    'myplot(1:10, [3 5 2 8 4 6 9 1 7 5]);'; ...
+```
+
+The gallery creates the figure for you, so the code just has to draw into it.
+Use fixed data (or seed the RNG) so the picture is reproducible, and keep the
+entry working in both engines when possible. Iterate on a single entry with:
+
+```matlab
+makegallery('Functions', {'myplot'}, 'Open', false)
+```
+
+Keep commits to one logical change with a short imperative message.