static: add readme

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

static/README.md

@@ -0,0 +1,49 @@
+# Building your own Docker static package
+
+Static packages can be built from this directory with the following syntax
+
+```console
+make TARGETPLATFORM=${TARGETOS}/${TARGETARCH}/${TARGETVARIANT} build
+```
+
+Format of `TARGETOS`, `TARGETARCH`, `TARGETVARIANT` is the same as the [platform ARGs in the global scope](https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope)
+for a Dockerfile like `linux/arm/v7`.
+
+Artifacts will be located in `build` under the following directory structure:
+`build/$os/$arch/$variant/` or `build/$os/$arch/` if there is no variant being
+used.
+
+### Building from local source
+
+Specify the location of the source repositories for the engine and cli when
+building packages
+
+* `ENGINE_DIR` -> Specifies the directory where the engine code is located, eg: `$GOPATH/src/github.com/docker/docker`
+* `CLI_DIR` -> Specifies the directory where the cli code is located, eg: `$GOPATH/src/github.com/docker/cli`
+
+```shell
+make ENGINE_DIR=/path/to/engine CLI_DIR=/path/to/cli TARGETPLATFORM=linux/amd64 build
+```
+
+## Supported platforms
+
+Here is a list of platforms that are currently supported:
+
+```console
+make TARGETPLATFORM=linux/amd64 build
+make TARGETPLATFORM=linux/arm/v6 build
+make TARGETPLATFORM=linux/arm/v7 build
+make TARGETPLATFORM=linux/arm64 build
+make TARGETPLATFORM=darwin/amd64 build
+make TARGETPLATFORM=darwin/arm64 build
+make TARGETPLATFORM=windows/amd64 build
+```
+
+> note: `darwin` only packages the docker cli and plugins.
+
+But you can test building against whatever platform you want like:
+
+```console
+make TARGETPLATFORM=linux/riscv64 build
+make TARGETPLATFORM=linux/s390x build
+```