readme: add examples and mention gosu

README.md

@@ -1,2 +1,22 @@
 # su-exec
-switch user and exec
+switch user and group id, setgroups and exec
+
+This is a simple tool that will simply execute a program with different
+privileges. The program will not run as a child, like su and sudo, so we
+work around TTY and signal issues.
+
+
+```console
+$ docker run -it --rm alpine:edge su -c 'ps aux'
+PID USER TIME COMMAND
+ 1 root 0:00 ash -c ps aux
+ 12 root 0:00 ps aux
+$ docker run -it --rm -v $PWD/su-exec:/sbin/su-exec:ro alpine:edge su-exec root ps aux
+PID USER TIME COMMAND
+ 1 root 0:00 ps aux
+```
+
+## Why reinvent gosu?
+
+This does more or less exactly the same thing as `gosu` but it is only 10kb instead of 1.8MB.
+