Add post-processing guides

[originalnicodr]

We could use some guides about different not-so-specific post-processing techniques that can help people out:

• Grain: A guide about how to use grain effectively, It should cover how grain adds detail in textures that lack some, why we want to turn off grain when shooting, different types of movie grain, why you may want to use grain textures instead of camera raw/lightroom grain, its effectiveness against banding, etc.

If a guide is added through a PR, please check the checkbox in this issue.