Add BTU modification (#18)

* Add files for Ploopy top without bearing sockets

- These files were created in Rhino by deleting the surfaces that made
  up the bearing sockets and patching the holes that were left
- This is a good base for the BTU top or any other mods people want to
  make

* Add BTU mod top piece v3

- This is the first full version of the BTU mod I created.
  - v1 and v2 were just test pieces that mounted to the front screw
    holes of the trackball to check clearances. They weren't really a
    usable top as they didn't have anything other than the ball well.
- This part has been test printed on an Ender 3 and works well.

* Rename STEPs folder to STEP

* Add BTU mod v4

- This is a full remake compared to v3, which was designed in Rhino. To
  make it more accessible for contributions, it's been redone in
  FreeCAD. This means you can edit it with a free program as well as
  making it parametric. The positions of the BTUs can be edited by
  changing the values in the spreadsheet embedded in the CAD file.
- These parts haven't yet been test printed

* Add README for BTU mod

* Add more detail to mod README

- Print and assembly tips
- Some images to show what the mod actually is and what to cut off the
  secondary buttons

* Rearrange bearing distributor list

- It didn't need to take up so many lines
- Also added Bosch's contact locator tool to make it easier for people
  to find other distributors, as suggested by /u/d4baller

hardware/Mechanicals-BTU-Mod/README.md

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+# Ploopy BTU Mod
+
+The roller bearings used in the Ploopy Trackball are nice, but with the standard ball they might not feel as smooth or as low-friction as you'd like. The prompt for this mod was "what if we replaced them with ball transfer units?"
+
+It turns out - good things happen!
+
+Ball transfer units (or BTUs) are omnidirectional bearings, as opposed to the single-direction roller bearings the Ploopy Trackball uses by default. This means you're never rolling the ball in the "wrong direction" for one of them.
+
+This mod is a new 3D-printable top piece for the Ploopy which contains holes for ball transfer units instead of roller bearings.
+
+Designer: [George Bryant](https://github.com/gbrnt) with help and bearing advice from [/u/d4baller](https://reddit.com/user/d4baller)
+
+## Versions
+### v3
+This version was designed in Rhino and so isn't really editable for most people. It works well though.
+
+![A trackball with this mod v3 installed](example-v3.jpg)
+
+### v4
+This version was redesigned in FreeCAD and the various dimensions are now editable in the FreeCAD file.
+
+Currently untested! It might collide with something internally, particularly if you've soldered in a programming header for the microcontroller.
+
+## Buying the BTUs
+The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or KU-B8-OFK. A set of three is about 20-30 USD, depending on where in the world you are and where you buy them from. Here are some sources we've found so far:
+
+* [Bosch-Rexroth](https://www.boschrexroth.com/en/xc/products/product-groups/linear-motion-technology/ball-transfer-units-and-tolerance-rings/ball-transfer-units/r0530-with-sheet-steel-housing/r0530-1-with-sheet-steel-housing) - UK and worldwide, but requires you to be a company (e.g. have a VAT number)
+* [Livhaven](https://store.livhaven.com/r053010810-bosch-rexroth-ball-transfer-unit) - US-based. Doesn't require company information, but overseas shipping is expensive.
+* [L-System](https://l-system.nu/webshop/linear-technology/ball-transfer-units-and-tolerance-rings/ball-transfer-units/type-r0530/ball-transfer-unit-ku-b8-ofk/) - Sweden-based. Might allow private customers to buy.
+* [Spareparts](https://www.sp-spareparts.com/en/p/r053010810-bosch-rexroth) - Ireland-based. Ships to UK and maybe elsewhere. Does sell to private customers.
+* [Rodivago](https://rodavigo.net/es/p/bola-transportadora-con-carter-de-metal-laminado-ref-053010810/557053010810) - Spain-based. May require being a company.
+* For other sources, try Bosch-Rexroth's [contact locator tool](https://addresses.boschrexroth.com/) - find a distributor near you, and search for the bearing part number on their website.
+
+## Printing
+With a speed of 50-60mm/s, support on the build plate and a layer height of 0.3mm you should get a print time of ~12.5 hours. It may be worth tweaking various settings to reduce it further.
+
+Highly recommended to enable support interface and splitting the support into chunks, as otherwise it can be very hard to remove.
+
+## Assembly
+* Remove all support material from the print.
+* Inspect the holes the BTUs will sit in to make sure there are no lumps inside the hole. File/sand/cut them off if needed.
+* Install the BTUs by pushing them into the holes.
+* Check the BTUs are fully installed by putting the ball on top. It shouldn't scrape on the top surface. If it did one or more of the BTUs might not be fully pushed in.
+* Remove the original top piece from your Ploopy trackball.
+* Remove the secondary button piece, grind off the stop (see below) and reinstall.
+* Install the new top piece on the trackball.
+* Give it a try!
+
+### Modifying secondary buttons
+There is a small tab on the secondary buttons which collides with the closest BTU. It needs to be sanded, filed or cut off. It is indicated in red in the image below:
+
+![Ploopy secondary buttons, with the tab on the end highlighted in red](trim-tab.jpg)