custom: split config for Custom AMI off

Up until now we have been repurposing the
Linux mbedtls AMI for use as "Custom AMI" to
run generic jobs that aren't necessarily
related to mbedtls.

This has led to some versioning problems as it's
not clear which version of an AMI is serving which
role.

Let's just split the custom AMI config off into its
own template so we can easily manage all these images
through this repo.

Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Change-Id: I582c1f8641d8bbe56e50714749395a615932c2e2
diff --git a/README.txt b/README.txt
new file mode 100644
index 0000000..fed29bc
--- /dev/null
+++ b/README.txt
@@ -0,0 +1,6 @@
+These are the packer build templates we use to create the AMIs
+used by the TrustedFirmware CI system.
+
+packer_freebsd.json -- produces the mbedtls FreeBSD AMI
+packer_ubuntu.json -- produces the mbedtls Linux AMI
+packer_custom.json -- produces an Ubuntu based utility AMI