Add readme, reorganize folder structure, add retro round dot shadow mask and NECT slot mask (#2)

* Add a readme so people know how the shadowmasks work.
* Reorganize folder structure to be similar to other repos in the project like this.
* Add retro (60's/70's) style CRT shadow mask which used a delta gun and round holes in the mask.
* Add a variant of the dot mask above that is brighter which uses MYC colors for people with much darker LCD tv's with CE backlight dimming.
* Add @wickerwaka's NECT slot mask he made awhile ago which resembles a popular style of shadow mask.

README.md

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+# Shadow Masks for [MiSTer FPGA](https://github.com/MiSTer-devel/Main_MiSTer/wiki)
+
+This repository contains the Shadow Mask files for the MiSTer FPGA.
+
+## Instructions
+
+Create a folder named `Shadow_Masks` in the root of your MicroSD Card along with the other folders like "filters", "games", etc... Place the .txt Shadow Mask files into that folder.
+
+## Color Palette
+
+| Color | Value | Bits |
+| --- | --- | --- |
+| Gray | 0 | 000 |
+| Blue | 1 | 001 |
+| Green | 2 | 010 |
+| Cyan | 3 | 011 |
+| Red | 4 | 100 |
+| Magenta | 5 | 101 |
+| Yellow | 6 | 110 |
+| White | 7 | 111 |
+
+Max width = 8  
+Max Height = 8
+
+## How It Works
+
+Here's an example. If you wanted to make RGB dots that use this pattern:
+
+![](example.png)
+
+Then you would make a file like this:
+
+```
+# w,h
+6,4
+
+# lut
+4,4,2,2,1,1
+4,4,2,2,1,1
+2,1,1,4,4,2
+2,1,1,4,4,2
+```
+
+These numbers correspond to the following r, g, b values like this:
+
+```
+r,r,g,g,b,b
+r,r,g,g,b,b
+g,b,b,r,r,g
+g,b,b,r,r,g
+```

Shadow_Masks/Misc/NECT Slot Mask.txt

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+# w,h
+8,6
+
+# lut
+4,2,1,0,4,2,1,0
+4,2,1,0,4,2,1,0
+4,2,1,0,0,0,0,0
+4,2,1,0,4,2,1,0
+4,2,1,0,4,2,1,0
+0,0,0,0,4,2,1,0
+

Shadow_Masks/Misc/Round Dot Mask - Brighter.txt

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+# w,h
+6,4
+
+# lut
+3,0,5,0,6,0
+0,0,0,0,0,0
+0,6,0,3,0,5
+0,0,0,0,0,0

Shadow_Masks/Misc/Round Dot Mask.txt

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+# w,h
+6,4
+
+# lut
+4,0,2,0,1,0
+0,0,0,0,0,0
+0,1,0,4,0,2
+0,0,0,0,0,0
+