Updated EnvironmentMapEffect (markdown)

EnvironmentMapEffect.md

@@ -78,6 +78,25 @@ This effect always performs texturing, so if 'untextured' rendering is desired y
 
 Both ``EffectFlags::Lighting`` and ``EffectFlags::Texture`` are always enabled for this effect, so use or absence of these flags are ignored for this effect.
 
+## Cubic Environment Map
+
+![Cubic Environment Map](https://github.com/Microsoft/DirectXTK/wiki/images/cubicenvmap.png)
+
+See [Microsoft Docs](https://docs.microsoft.com/en-us/windows/win32/direct3d9/cubic-environment-mapping)
+
+## Spherical Environment Map
+
+![Sphere Environment Map](https://github.com/Microsoft/DirectXTK/wiki/images/sphereenvmap.png)
+
+See [Microsoft Docs](https://docs.microsoft.com/en-us/windows/win32/direct3d9/spherical-environment-mapping)
+
 # Further reading
 
 [EnvironmentMapEffect]( http://www.shawnhargreaves.com/blog/environmentmapeffect.html)  
+
+Blinn & Newell, "Texture and Reflection in Computer Generated Images", _Communications of the ACM_. 1976.
+[link](https://www.microsoft.com/en-us/research/wp-content/uploads/1976/10/p542-blinn.pdf)
+
+Greene, "Environment Mapping and Other Applications of World Projections", _IEEE Computer Graphics and Applications_., 1986. [link](https://ieeexplore.ieee.org/document/4056759)
+
+Heidrich & Seidel, “View-independent Environment Maps”, _Eurographics Workshop on Graphics Hardware_, 1998. [link](https://diglib.eg.org/bitstream/handle/10.2312/EGGH.EGGH98.039-045/039-045.pdf)