add Dual Rail Installation Guide (#379)

printer_mods/README.md

@@ -100,6 +100,7 @@ like so:
 |  | [Voron 2.4 cable clamps](./evandepol/Voron2.4-Cable-clamps) | A set of 2020-mounted cable guards to organize and dress the wiring on your build | :x: | :x: | :heavy_check_mark: |:x:|
 | FreeformFez | [V1.8 Filtered Power Inlet](./FreeformFez/FilteredPower) | V1.6 Filtered Inlet and Rocker Switch for V1.8 | :x: | :heavy_check_mark: | :x: |:x:|
 | geoffreyyoung | [FYSETC SPIDER DIN Bracket](./geoffreyyoung/spider_bracket) | FYSETC SPIDER DIN Bracket | :x: | :heavy_check_mark: | :heavy_check_mark: | :x: |
+| | [Dual Rail Installation Guide](./geoffreyyoung/dual_rail_guide) | Rail installation guide that also works for the 2.4 dual rail installation | :x: | :x: | :heavy_check_mark: | :x: |
 | GiulianoM | [Fly F407ZG Mount](./GiulianoM/FlyF407ZG) | Mount for the Fly F407ZG: https://github.com/FLYmaker/FLYF407ZG | :x: | :x: | :heavy_check_mark: | :x: |
 | Gizzle | [Ender 3 and Ender 3 Pro Switchwire Mod](./Gizzle/ender-3_(pro)_switchwire)| Mod for the Ender 3 and Ender 3 Pro, which adapts the kinematics and most other parts of the Voron Switchwire. | :x: | :x: | :x: | :heavy_check_mark: |
 | GSL12 | [Pocketwatch Nema 17](./GSL12/pocketwatch_nema17)| Extruder like Pocketwatch, but with Nema17 instead of Nema14. Maybe for V0 fork with 2020 extrusions, Nema17 and MGN9 or other printers. | :grey_question: | :grey_question: | :grey_question: |:x:|

printer_mods/geoffreyyoung/README.md

@@ -4,7 +4,8 @@
 
 #### in [VoronUsers](https://github.com/VoronDesign/VoronUsers)...
 
-* [`spider_bracket.stl`](https://github.com/geoffrey-young/VoronUsers/tree/geoffrey-young-spider_bracket/printer_mods/geoffreyyoung/spider_bracket) - Minimalist DIN bracket for the [FYSETC SPIDER](https://wiki.fysetc.com/Spider/), based on the simple design of the core SKR bracket.  It uses a [standard build `pcb_din_clip_x3.stl`](https://github.com/VoronDesign/Voron-2/blob/Voron2.4/STLs/VORON2.4/Electronics_Compartment/DIN_Brackets/pcb_din_clip_x3.stl), and provides integrated horizontal and vertical mount options so you can choose on the fly.
+* [Dual Rail Installation Guide](./dual_rail_guide) - Guide for centering the Voron 2.4 dual X rails during installation.
+* [FYSETC SPIDER DIN Bracket](./spider_bracket) - Minimalist DIN bracket for the [FYSETC SPIDER](https://wiki.fysetc.com/Spider/), based on the simple design of the core SKR bracket.  It uses a [standard build `pcb_din_clip_x3.stl`](https://github.com/VoronDesign/Voron-2/blob/Voron2.4/STLs/VORON2.4/Electronics_Compartment/DIN_Brackets/pcb_din_clip_x3.stl), and provides integrated horizontal and vertical mount options so you can choose on the fly.
 
 ---
 

printer_mods/geoffreyyoung/dual_rail_guide/README.md

@@ -0,0 +1,13 @@
+# Dual Rail Installation Guide
+
+A guide for centering the Voron 2.4 dual X rails during installation.
+
+The guide is identical to the [core Voron 2.4 rail installtion guide](https://github.com/VoronDesign/Voron-2/blob/Voron2.4/STLs/VORON2.4/Tools/rail_installation_guide_center_x2.stl), with the top extruded another 10mm longer so it can touch the face of the extrusion even after the first rail is installed.  If that doesn't make sense, see the picture... or just print it and you'll see :)
+
+## Printing
+
+Use the standard Voron 2.4 recommended print settings.  As with the core part, you will want to print two.  If you choose to use this you can skip printing the core guide - you don't need both.
+
+## Images
+
+![guide.jpg](images/guide.jpg)