add readme

README.md

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+# infinitime-builder
+
+Build InfiniTime firmware images using [nix](https://nixos.org/).
+
+## Building
+
+```shell
+nix --experimental-features 'nix-command flakes' build
+```
+
+The images to flash will be put into `./result`
+
+(If you already have flakes enabled you can skip the `--experimental-features` bit)
+
+## Customizing
+
+You can customize the build image by editing `package.nix` and adding them to the `patches` array like:
+
+```nix
+patches = [
+  # grap the patch directly from github
+  (fetchpatch {
+    name = "double-click-settings.patch";
+    url = "https://github.com/InfiniTimeOrg/InfiniTime/commit/d395f6f0857f082ab3e7fc66cb591b12bbd9cd65.patch";
+    sha256 = "sha256-2Jng8JDTU7Zd42qD25CQNeF9dQRSLNwWq9IL+ik0ok0=";
+  })
+  # or from a local file (don't forget to add the patch to git before building)
+  ./local-patch.patch
+]
+```
+
+## Updating
+
+To change the version being build you can edit the `src` directive with a different `rev` and `hash`.
+
+You can determine the correct `hash` for a revision by running
+
+```shell
+nix --experimental-features 'nix-command flakes' run github:nix-community/nurl -- https://github.com/InfiniTimeOrg/InfiniTime <revision> --submodules
+```