efilinux: Explain the config file syntax and add an example

The only explanation of the config file syntax is in the commit log
that introduced it, commit 77148a3e4978 ("efilinux: Minimal
configuration file support"), so add a couple of lines to the README
describing it. Also, add an example config file.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>
diff --git a/README b/README
index 6399d82..05ed9df 100644
--- a/README
+++ b/README
@@ -14,5 +14,10 @@
+There is no config syntax as such. A config file is a one-line file
+that contains command line parameters. See example.cfg.
 Matt Fleming <matt.fleming@intel.com>
diff --git a/example.cfg b/example.cfg
new file mode 100644
index 0000000..bc3f786
--- /dev/null
+++ b/example.cfg
@@ -0,0 +1 @@
+-f 0:\bzImage console=ttyS0 initrd=0:\initrd