playbook: Update README.md

playbook/roles/install-script/README.md

-
 The installation scripts are splited on 4 diferent keys or channels:
 
-  slapos-global-key: Fully stable playbook, Dedicated to KVM cluster autosetup images and
-             computers who host KVM clusters on IMT infraestructure.
+- `slapos-global-key` Fully stable playbook, Dedicated to KVM cluster autosetup images and 
+    computers who host KVM clusters on IMT infraestructure.
 
-  slapos-global-unstable-key: Unstable or Development playbook alternative for 
-             slapos-global-key.
+- `slapos-global-unstable-key` Unstable or Development playbook alternative for 
+    slapos-global-key.
 
- 
-  slapos-global-vifib-official-key: Official VIFIB and Grandenet Stable Playbook, it include
-            either stable versions of the Cluster/Standalone servers. 
+- `slapos-global-vifib-official-key` Official VIFIB and Grandenet Stable Playbook, it include
+    either stable versions of the Cluster/Standalone servers. 
              
-
-  slapos-global-vifib-testing-key: Testing Playbook VIFIB and Grandenet Stable Playbook, it 
-          include unstable (or more recently versions) of the Cluster/Standalone servers.  
+- `slapos-global-vifib-testing-key` Testing Playbook VIFIB and Grandenet Stable Playbook, it 
+    include unstable (or more recently versions) of the Cluster/Standalone servers.  
 
 
 Developers channels (or keys) are not included on this list, only official ones.