added building instructions

building.md

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+# Building PCBs
+
+In order to get PCBs made, you usually need to provide them with the following files;
+
+- Gerbers
+- Drill Files
+
+Additionally if you want to have a shop assemble the boards for you, you’ll also need to provide;
+
+- Bill of Materials (BOM)
+- Centroid file (XY)
+
+### Generating Gerbers and Drill Files
+
+Gerbers are a set of files in a standard format that allow a PCB manufacturer to build the PCB. Usually there is one gerber file for each layer of a PCB. A Gerber file can represent a wide variety of different layer types;
+
+- Copper layer
+- Silkscreen Layer
+- Soldermask layer
+- Board outline
+- Drills (Okay, this is actually not a gerber, but your PCB house will definitely need it)
+
+Any* PCB CAD software will allow you to generate gerbers. We use [KiCad](https://www.kicad.org) because it is open source, free, and *awesome*. Here is how to generate gerbers in KiCad v6;
+
+1. Open up the design in KiCad and switch to the PCB board editor view (ie not the schematic). Some people call this pcbnew.
+2. Choose File → Fabrication Outputs → Gerbers (.gbr)…
+3. Pick the right options for what your PCB board house expects. Here is what I use;
+
+![Gerber Plot Window](doc/gerbers.png)
+
+Make sure you have these “Include Layers” checked;
+
+- **F.Cu** → Front copper
+- **In1.Cu** → Inner layer 1 Copper
+- **In2.Cu** → Inner layer 2 Copper
+- **B.Cu** → Back copper
+- **F.Paste** → Front Paste. Note: this is used to make a solder paste stencil. This is optional. Many PCB houses will let you order stencils as a separate step. use these gerbers for ordering a stencil
+- **B.Paste** → Back paste. see note above
+- **F.Silkscreen** → Front Silkscreen. sometimes this is called the Legend
+- **B.Silkscreen** → Back Silkscreen. sometimes this is called the Legend of Zelda
+- **F.Mask** → Front soldermask
+- **B.Mask** → Back soldermask
+- **Edge.Cuts** → The board outline. This defines the shape the PCB is cut to.
+
+1. Next you’ll want to generate the drill files. Click that button in the lower right corner of the “Plot” screen from above.
+2. Generate the drill files according to your PCB house specs. Here are the options I use;
+
+![Drill Files Window](doc/drill.png)
+
+That’s it! Take all of those generated files and give them to your PCB house. If you’re ordering stencils (which you should!), pull out `-F_Paste.gbr` and `-B_Paste.gbr` and send those to the right place. Paste gerbers are not needed to manufacture a PCB.
+
+### Generating PCBA Manufacturing files
+
+- TBD