feat(hw): Add mnemos shield (#163)

This PR adds the CAD design files for the second version of the mnemos
shield. This design matches the version of the boards that were built
for @hawkw.

The original revision had the center row of headers with 1.2mm PTH,
which were too large to be functional for the pogo pins. This history is
not included (because it didn't fully work).

Gerber exports, suitable for ordering from JLC or other board fabs will
be added as a github release after this has been merged.

hardware/mnemos-shield/README.md

@@ -0,0 +1,18 @@
+# Mnemos Shield
+
+![A render of the layout of the mnemos shield PCB](../../assets/hw-mnemos-shield-v2.png)
+
+This is a basic breakout board for the beepy hardware, to overcome the fact that the MQ-Pro PTH are too small for the pogo pins used by the beepy board.
+
+This board contains three 40-pin headers all connected in parallel with each other. All holes are 1.0mm inner diameter, which seems to work well on the pogo pins.
+
+## Usage Warning
+
+At the moment, it's not a *great* idea to power a beepy+MQ-Pro at the same time.
+
+It's recommended (if you build one of these boards) to either:
+
+* Don't connect the header pins for the MQ-Pro 5v0 rails
+* Cut the traces on the shield
+
+This will likely be fixed in a future revision. If you are planning to order boards from these design files, please drop by [#mnemos-dev:beeper.com](https://matrix.to/#/#mnemos-dev:beeper.com) so we can help you out!