Add a shields readme

boards/shields/README.md

@@ -0,0 +1,13 @@
+## Custom shields
+
+This repo also contains shield definitions for a few keyboards. Ones using Pro Micro format daughterboards:
+
+- [`choc_ergo`](https://keypcb.xyz/choc_ergo)
+- [`hypergolic`](https://github.com/davidphilipbarr/hypergolic) -- you should probably use [the official Cradio shield](https://github.com/zmkfirmware/zmk/tree/main/app/boards/shields/cradio/) instead
+- [`ffkb` v1](https://fingerpunch.xyz/product/faux-fox-keyboard/) with an OLED, building upon [sadekbaroudi's](https://github.com/sadekbaroudi/zmk-ffkb) and [NCKiser's](https://github.com/NCKiser/zmk-ffkb) definitions
+- [`swweeep`](https://github.com/sadekbaroudi/sweep36) using [sadekbaroudi's](https://github.com/sadekbaroudi/zmk-swweeep) definition
+
+Ones using Seeeduino Xiao format daughterboards:
+
+- [`rommana`](https://github.com/AlaaSaadAbdo/rommana): A hand-wired keywell keyboard with 28 keys, using Xiao BLEs
+- [`grumpybird`](https://github.com/caksoylar/Grumpy/tree/hummingbird-pinky): A hand-wired variant of the [Grumpy](https://github.com/weteor/Grumpy) keyboard, where upper pinky rather than lower pinky is used