Created Default shader parameters (markdown)

Default-shader-parameters.md

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+The default shaders included in raylib defines the following default input and output parameters if no user-provided vertex or fragment shader is provided.
+
+Source: [rlgl.h](https://github.com/raysan5/raylib/blob/master/src/rlgl.h)
+
+## Default vertex shader
+
+OPENGL_ES2 or OPENGL_21
+```
+attribute vec3 vertexPosition;
+attribute vec2 vertexTexCoord;
+attribute vec4 vertexColor;
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+```
+
+OPENGL_33
+```
+in vec3 vertexPosition;
+in vec2 vertexTexCoord;
+in vec4 vertexColor;
+out vec2 fragTexCoord;
+out vec4 fragColor;
+```
+
+## Default fragment shader
+
+OPENGL_ES2 or OPENGL_21
+```
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+```
+OPENGL_ES2 also defines
+```
+precision mediump float;
+```
+
+OPENGL_33
+```
+in vec2 fragTexCoord;
+in vec4 fragColor;
+out vec4 finalColor;
+```