Update README

Add usage of build script

README.md

@@ -8,7 +8,7 @@ so you must build your own packer.
 Here are some examples steps to do this for a linux box.
 
 Install Go to build packer:
-```
+```shell
 wget https://dl.google.com/go/go1.12.7.linux-amd64.tar.gz
 mkdir ~/opt/go
 tar -C ~/opt/go -xzf go1.12.7.linux-amd64.tar.gz
@@ -16,10 +16,28 @@ PATH=~/opt/go/bin:$PATH
 ```
 
 Build packer:
-```
+```shell
 mkdir -p ~/go/src/github.com/hashicorp && cd $_
 git clone https://github.com/hashicorp/packer.git
 cd packer
 make dev
 PATH=~/go/src/github.com/hashicorp/packer/bin:$PATH
 ```
+
+Building a compute image:
+
+- This build script will handle floating ip, ssh host for you. All you need to change is user define section in the `build-compute.py`.
+
+```shell
+# Get your openstack credential
+source app-cred.sh
+
+# Activate your openstack (venv)
+source PATH/TO/YOUR/VENV
+
+# Change user defined section in build script if needed
+vim build-compute.py
+
+# Run build script
+python build-compute.py
+```