update doc

Author: mazznoer

PRESET.md

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+# Preset Gradients
+
+All preset gradients are in the domain [0..1].
+
+## Diverging
+
+`colorgrad.BrBG()`
+![img](doc/images/preset/BrBG.png)
+
+`colorgrad.PRGn()`
+![img](doc/images/preset/PRGn.png)
+
+`colorgrad.PiYG()`
+![img](doc/images/preset/PiYG.png)
+
+`colorgrad.PuOr()`
+![img](doc/images/preset/PuOr.png)
+
+`colorgrad.RdBu()`
+![img](doc/images/preset/RdBu.png)
+
+`colorgrad.RdGy()`
+![img](doc/images/preset/RdGy.png)
+
+`colorgrad.RdYlBu()`
+![img](doc/images/preset/RdYlBu.png)
+
+`colorgrad.RdYlGn()`
+![img](doc/images/preset/RdYlGn.png)
+
+`colorgrad.Spectral()`
+![img](doc/images/preset/Spectral.png)
+
+## Sequential (Single Hue)
+
+`colorgrad.Blues()`
+![img](doc/images/preset/Blues.png)
+
+`colorgrad.Greens()`
+![img](doc/images/preset/Greens.png)
+
+`colorgrad.Greys()`
+![img](doc/images/preset/Greys.png)
+
+`colorgrad.Oranges()`
+![img](doc/images/preset/Oranges.png)
+
+`colorgrad.Purples()`
+![img](doc/images/preset/Purples.png)
+
+`colorgrad.Reds()`
+![img](doc/images/preset/Reds.png)
+
+## Sequential (Multi-Hue)
+
+`colorgrad.Turbo()`
+![img](doc/images/preset/Turbo.png)
+
+`colorgrad.Viridis()`
+![img](doc/images/preset/Viridis.png)
+
+`colorgrad.Inferno()`
+![img](doc/images/preset/Inferno.png)
+
+`colorgrad.Magma()`
+![img](doc/images/preset/Magma.png)
+
+`colorgrad.Plasma()`
+![img](doc/images/preset/Plasma.png)
+
+`colorgrad.Cividis()`
+![img](doc/images/preset/Cividis.png)
+
+`colorgrad.Warm()`
+![img](doc/images/preset/Warm.png)
+
+`colorgrad.Cool()`
+![img](doc/images/preset/Cool.png)
+
+`colorgrad.CubehelixDefault()`
+![img](doc/images/preset/CubehelixDefault.png)
+
+`colorgrad.BuGn()`
+![img](doc/images/preset/BuGn.png)
+
+`colorgrad.BuPu()`
+![img](doc/images/preset/BuPu.png)
+
+`colorgrad.GnBu()`
+![img](doc/images/preset/GnBu.png)
+
+`colorgrad.OrRd()`
+![img](doc/images/preset/OrRd.png)
+
+`colorgrad.PuBuGn()`
+![img](doc/images/preset/PuBuGn.png)
+
+`colorgrad.PuBu()`
+![img](doc/images/preset/PuBu.png)
+
+`colorgrad.PuRd()`
+![img](doc/images/preset/PuRd.png)
+
+`colorgrad.RdPu()`
+![img](doc/images/preset/RdPu.png)
+
+`colorgrad.YlGnBu()`
+![img](doc/images/preset/YlGnBu.png)
+
+`colorgrad.YlGn()`
+![img](doc/images/preset/YlGn.png)
+
+`colorgrad.YlOrBr()`
+![img](doc/images/preset/YlOrBr.png)
+
+`colorgrad.YlOrRd()`
+![img](doc/images/preset/YlOrRd.png)
+
+## Cyclical
+
+`colorgrad.Rainbow()`
+![img](doc/images/preset/Rainbow.png)
+
+`colorgrad.Sinebow()`
+![img](doc/images/preset/Sinebow.png)