Add unix install instrux for terraform

Add steps for installing terraform on unix systems using the
unzip method.
Add additional text describing install and labling existing
mac steps.

README.md

@@ -19,8 +19,21 @@ will connect instance to ood through ohpc and download software
 
 ### Install Terraform:
 
+Install terraform from your systems package manager or [directly from HashiCorp](https://learn.hashicorp.com/terraform/getting-started/install.html).
+Terraform is a single binary so it's easy to maintain the latest release version
+from the distributed zip files.
+
+On a Mac (using brew package manager):
+
 - `$brew install terraform`
 
+On a linux box (using the upstream package installed to personal
+ ~/bin already in the $PATH):
+```
+wget https://releases.hashicorp.com/terraform/0.12.3/terraform_0.12.3_linux_amd64.zip
+unzip terraform_0.12.3_linux_amd64.zip -d ~/bin
+```
+
 ### Unzip the file and make a path to your new directory using:
 
 - `$export TERRAFORM_DIR="$(pwd)/terraform-first-instance`