Add new custom hypervisor type docs

source/installguide/hypervisor/custom.rst

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+
+Custom Hypervisor Installation
+------------------------------
+
+CloudStack allows adapting a new hypervisor plugin that may be developed privately.
+
+The new hypervisor type is internally named Custom to the CloudStack services.
+
+
+The setting ‘hypervisor.custom.display.name’ allows administrators to set the display name of the hypervisor type.
+The display name will be shown in the CloudStack UI and API.
+
+    - In case the ‘hypervisor.list’ setting contains the display name of the new hypervisor type, the setting value is automatically updated after the ‘hypervisor.custom.display.name’ setting is updated.
+
+
+The custom hypervisor type supports:
+
+    - Direct downloads (the ability to download templates into primary storage from the hypervisor hosts without using secondary storage)
+    - Local storage (use hypervisor hosts local storage as primary storage)
+    - Template format: RAW format (the templates to be registered on the new hypervisor type must be in RAW format)
+
+The UI is also extended to display the new hypervisor type and the supported features listed above.