Update readme to include usage example

.gitignore

@@ -1,3 +1,6 @@
+# Vim swap files
+.*.swp
+
 # Object files
 *.o
 *.ko

README.md

@@ -1,22 +1,46 @@
 # su-exec
 switch user and group id, setgroups and exec
 
+## Purpose
+
 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.
+privileges. The program will be exceuted directly and not run as a child,
+like su and sudo does, which avoids TTY and signal issues (see below).
+
+Notice that su-exec depends on being run by the root user, non-root
+users do not have permission to change uid/gid.
+
+## Usage
+
+```shell
+su-exec user-spec command [ arguments... ]
+```
+
+`user-spec` is either a user name (e.g. `nobody`) or user name and group
+name separated with colon (e.g. `nobody:ftp`). Numeric uid/gid values
+can be used instead of names. Example:
+
+```shell
+$ su-exec apache:1000 /usr/sbin/httpd -f /opt/www/httpd.conf
+```
+
+## TTY & parent/child handling
 
+Notice how `su` will make `ps` be a child of a shell while `su-exec`
+just executes `ps` directly.
 
-```console
-$ docker run -it --rm alpine:edge su -c 'ps aux'
+```shell
+$ docker run -it --rm alpine:edge su postgres -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 $PWD/su-exec:/sbin/su-exec:ro alpine:edge su-exec  root ps aux
+    1 postgres   0:00 ash -c ps aux
+   12 postgres   0:00 ps aux
+$ docker run -it --rm -v $PWD/su-exec:/sbin/su-exec:ro alpine:edge su-exec postgres ps aux
 PID   USER     TIME   COMMAND
-    1 root       0:00 ps aux
+    1 postgres   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.
+This does more or less exactly the same thing as [gosu](https://github.com/tianon/gosu)
+but it is only 10kb instead of 1.8MB.