add README with initial instructions

README.md

@@ -0,0 +1,35 @@
+# u-root builder
+
+This repository holds no code and just builds [u-root](https://u-root.org)
+images via GitHub Actions.
+
+## Try it out
+
+Download a prebuilt u-root image from
+[releases](https://github.com/linuxboot/u-root-builder/releases):
+
+```sh
+curl -o u-root.cpio.xz https://github.com/linuxboot/u-root-builder/releases/download/v0.0.1/u-root_amd64_all.cpio.xz
+```
+
+Download a
+[kernel from Arch Linux](https://archlinux.org/packages/core/x86_64/linux/)
+and extract it:
+
+```sh
+curl -L -o linux.tar.zst https://archlinux.org/packages/core/x86_64/linux/download/
+tar -xf linux.tar.zst
+```
+
+Run it:
+
+```sh
+qemu-system-x86_64 -enable-kvm -machine q35 -nographic -append "console=ttyS0" \
+  -kernel usr/lib/modules/*/vmlinuz -initrd u-root.cpio.xz
+```
+
+Run a command:
+
+```sh
+uname -a
+```