Add demo descriptions via README files

Signed-off-by: Tobias Frisch <tfrisch@uni-koblenz.de>

demos/CubeMapping/README.md

+# CubeMapping
+A demo project to show how to implement cube maps with the VkCV framework
+
+![Screenshot of CubeMapping](../../screenshots/CubeMapping.png)
+
+## Details
+
+Cube maps can be used to create the illusion of a big environment, render a highly detailed sky 
+around the camera or to create very efficient reflections on smooth surfaces.
+
+The cube map texture will contain six different images which are stored on different layers of the
+image resource in Vulkan. The shaders can later simply access the pixel data via a 3D directional
+vector.
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demos/InstancingDemo/README.md

+# InstancingDemo
+A demo project to show how to render meshes using instancing with the VkCV framework
+
+![Screenshot of InstancingDemo](../../screenshots/InstancingDemo.png)
+
+## Details
+
+Recording many draw calls can be expensive for your GPU because each draw call has to manage a lot
+of data to work properly. But in cases you want to render the same vertex data multiple times with
+only slight variations, you can use instancing to save up resource cost.
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demos/NormalMapping/README.md

+# NormalMapping
+A demo project to show how to use normal maps with the VkCV framework
+
+![Screenshot of NormalMapping](../../screenshots/NormalMapping.png)
+
+## Details
+
+Ideally we want to render highly detailed meshes with a GPU. But that can be very expensive in terms
+of processing power. It is much more efficient to load such details like normal vectors from a
+texture instead of using only interpolation between vertices to calculate them.
+
+So we get a lot of visual details without many vertices actually processed. It is a great tradeoff 
+between processing power and memory usage.
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demos/SSAO/README.md

+# SSAO
+A demo project to show how to use screen-space ambient occlusion with the VkCV framework
+
+![Screenshot of SSAO](../../screenshots/SSAO.png)
+
+## Details
+
+Using rasterization for rendering has one huge drawback. We usually don't have proper indirect 
+lighting. That also means that we don't get any ambient occlusion or indirect shadows for the 
+rendered scene.
+
+This project shows off a technique which utilizes deferred shading to calculate the amount of 
+occlusion by other geometry for each pixel. The technique will only use information from the 
+screen-space. So keep in mind it is limited in accuracy.
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