add documentation

docs/coreos.md

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+CoreOS bootstrap
+===============
+
+Example with **kargo-cli**:
+
+```
+kargo deploy --gce --coreos
+```
+
+Or with Ansible:
+
+Before running the cluster playbook you must satisfy the following requirements:
+
+* On each CoreOS nodes a writable directory **/opt/bin** (~400M disk space)
+
+* Uncomment the variable **ansible\_python\_interpreter** in the file `inventory/group_vars/all.yml`
+
+* run the Python bootstrap playbook
+
+```
+ansible-playbook -u smana -e ansible_ssh_user=smana  -b --become-user=root -i inventory/inventory.cfg coreos-bootstrap.yml
+```
+
+Then you can proceed to [cluster deployment](#run-deployment)