readme: add initial doc

README.md

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+`postmarketos-mkinitfs` is a tool for generating an initramfs (and installing
+it) on postmarketOS.
+
+## Building
+
+Building this project requires a Go compiler/toolchain and `make`:
+
+```
+$ make
+```
+
+To install locally:
+
+```
+$ make install
+```
+
+Installation prefix can be set in the generally accepted way with setting
+`PREFIX`:
+
+```
+$ make PREFIX=/some/location
+# make PREFIX=/some/location install
+```
+
+Other paths can be modified from the command line as well, see the top section of
+the `Makefile` for more information.
+
+Tests (functional and linting) can be executed by using the `test` make target:
+
+```
+$ make test
+```
+
+## Usage
+
+The application uses configuration from `/etc/deviceinfo`, and does not support
+any other options at runtime. It can be run simply by executing:
+
+```
+$ postmarketos-mkinitfs
+```
+
+For historical reasons, a symlink from `mkinitfs` to `postmarketos-mkinitfs` is
+also installed by the makefile's `install` target.