make documentation

[tinaok]

Please make documentation

here is the example how to use the code

import healpix_plotting
import cartopy.crs as ccrs

fig, axes = plt.subplots(1, 1, subplot_kw={"projection": ccrs.Mollweide()},figsize=(12,12))
mappable = healpix_plotting.plot(cell_ids, ar1, 
                                 healpix_grid={"level": child_level, "indexing_scheme": "nested", "ellipsoid":"WGS84"}, 
                                 sampling_grid={"shape": 400},ax=axes)
fig.colorbar(mappable, orientation="horizontal")
ax= mappable.figure.axes[0]
ax.coastlines()
ax.gridlines(draw_labels="x")
ax.gridlines(draw_labels="y")