[SCB-569] Add document for enabling SSL

[yangbor]

Add document for enabling SSL for communication between alpha and omega.

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[coveralls]

Coverage decreased (-0.6%) to 94.605% when pulling 3f02dcf on yangbor:master into d8ad37c on apache:master.

[coveralls]

Coverage decreased (-1.06%) to 94.107% when pulling 2c1b00f on yangbor:master into d8ad37c on apache:master.

[WillemJiang]

@yangbor Can we add a link in the user guide md?

[yangbor]

@WillemJiang Done