Add note on usage

README.md

@@ -14,6 +14,8 @@ should log to stdout/stderr).
 The only real requirement is a */run* tmpfs directory mounted with the
 *exec* flag.
 
+## Configuration
+
 The init sequence works as follows:
 
 * First, scripts in */etc/cont-init.d* are executed, in order. If any
@@ -53,7 +55,7 @@ fi
 exec foo
 ```
 
-### Build system
+## Build system
 
 The toolchain and build system is lifted straight from s6-overlay so
 it maintains some of the same properties (e.g. statically linked, can
@@ -69,3 +71,16 @@ However instead of preparing tar.xz files for distribution, we
 actually build a container image (based on Debian), and merge our
 *layout* directory at that stage. The whole build process is handled
 by the multi-stage Dockerfile.
+
+## Usage
+
+You can use the container image built by the CI for this repository as
+a base image for your own. E.g., in a Dockerfile:
+
+```
+FROM registry.git.autistici.org/ai3/docker/s6-overlay-lite:master
+...
+```
+
+The ENTRYPOINT is set to */init* by the base image and should not be
+modified.