README: Explain how to initialize and apply changes to infra

Related T1784

proxmox/terraform/README.md

+# What
+
+Terraform allows to transparently declare our infrastructure as code. Providing
+a (non-official so far) plugin, we can provision vm the same way for our rocq
+infra (proxmox)
+
+# The road so far
+
+## Prepare workstation
+
+See prepare-workstation.md
+
+## setup.sh
+
+Create a `setup.sh` file holding the PM_{USER,PASS} information:
+
+```
+export PM_USER=<swh-login>@pam
+export PM_PASS=<swh-login-pass>
+```
+
+source it in your current shell session.
+
+```
+source setup.sh
+```
+
+## provision new vm
+
+```
+terraform init
+terraform apply
+```
+
+# Details
+
+The provisioning is bootstraping vm declared in ".tf" files. It's using a base
+template (debian-9-template, debian-10-template) installed in the hypervisor.
+Instructions are detailed in the `init-template.md` file.
+
+# Init
+
+This initializes your local copy with the necessary:
+
+```
+terraform init
+```
+
+# Plan changes
+
+Compulse all *.tf files present in the folder, then compute a
+differential plan:
+
+```
+terraform plan
+```
+
+# Apply changes
+
+Propose to apply the plan to the infra (interactively):
+
+```
+terraform apply
+```

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T1784 is now infra/puppet/puppet-swh-site#1784