added the readme

shaders/deferred_lighting.frag

@@ -0,0 +1,61 @@
+#version 330 core
+out vec4 FragColor;
+
+in vec2 TexCoord;
+
+uniform sampler2D       gPosition;
+uniform sampler2D       gNormal;
+uniform sampler2D       gAlbedo;
+uniform sampler2D       gLightSpacePos;
+uniform sampler2DShadow shadowMap;
+
+struct DirLight {
+    vec3 direction;
+    vec3 ambient;
+    vec3 diffuse;
+    vec3 specular;
+};
+uniform DirLight dirLight;
+uniform vec3     viewPos;
+
+float ShadowCalculation(vec4 fragPosLightSpace)
+{
+    vec3 projCoords = fragPosLightSpace.xyz / fragPosLightSpace.w;
+    projCoords      = projCoords * 0.5 + 0.5;
+    if (projCoords.z > 1.0) return 0.0;
+    return texture(shadowMap, vec3(projCoords.xy, projCoords.z));
+}
+
+vec3 CalcDirLight(DirLight light, vec3 normal, vec3 viewDir, vec4 fragPosLS, vec3 albedo)
+{
+    vec3  lightDir   = normalize(-light.direction);
+    float diff       = max(dot(normal, lightDir), 0.0);
+    vec3  halfwayDir = normalize(lightDir + viewDir);
+    float spec       = pow(max(dot(normal, halfwayDir), 0.0), 32.0);
+
+    vec3 ambient  = light.ambient  * albedo;
+    vec3 diffuse  = light.diffuse  * diff * albedo;
+    vec3 specular = light.specular * spec;
+
+    float shadow = ShadowCalculation(fragPosLS);
+    return ambient + (diffuse + specular) * shadow;
+}
+
+void main()
+{
+    vec3 fragPos       = texture(gPosition,      TexCoord).rgb;
+    vec3 normal        = normalize(texture(gNormal, TexCoord).rgb);
+    vec4 albedaSample  = texture(gAlbedo,        TexCoord);
+    vec4 fragPosLS     = texture(gLightSpacePos, TexCoord);
+
+    // Sky (no geometry written) — output clear color
+    if (length(normal) < 0.1) {
+        FragColor = vec4(0.0, 0.0, 0.0, 0.0);
+        return;
+    }
+
+    vec3 viewDir = normalize(viewPos - fragPos);
+    vec3 lit     = CalcDirLight(dirLight, normal, viewDir, fragPosLS, albedaSample.rgb);
+
+    FragColor = vec4(lit, albedaSample.a);
+}