Updated Defining custom shaders (markdown)

Defining-custom-shaders.md

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+Custom shader definitions can be made by modifying the standard shader definitions, found in **shaders.txt**, or by adding new or updated definitions to the **custom-shaders.txt** file (the preferred method).
+
 There are currently three basic classes of shaders, any of which can be used to define a custom shader:
 
 - **org.dynmap.hdmap.DefaultHDShader** - this shader supports the equivalents of the FlatMap and KzedMap color models, including the use of colorschemes, and various biome-based coloring