Add basic README.

README.md

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+# Blank Slate ZMK Firmware
+
+This repo contains the firmware for the Blank Slate PCB.
+
+For full build guide for the Blank Slate, see https://docs.lpgala.xyz/docs/blank-slate-build-guide/overview/
+
+# Soft Off Support
+
+Blank Slate includes support for a ZMK feature this is still [in a PR](https://github.com/zmkfirmware/zmk/pull/1942). By default, this repository builds against that feature branch to ensure that works properly.
+
+Should you desire to track ZMK `main` (or some other branch of ZMK), simply edit the `config/west.yml` file and replace the contents with:
+
+```
+manifest:
+  remotes:
+    - name: zmkfirmware
+      url-base: https://github.com/zmkfirmware
+  projects:
+    - name: zmk
+      remote: zmkfirmware
+      revision: main
+      import: app/west.yml
+  self:
+    path: config
+```