Add a doc sample for how to use the required field
Add a doc sample for how to use the required field

This commit adds a sample to the developer documentation on setting
an option as required. Previously there was no example and it just
stated it could be done, which was a little confusing.

Change-Id: I1eb96422fd4ac8a83a5ac9397905b58fa7d6c8e0

JioCloud/oslo.config,magic0704/oslo.config,shad7/oslo.config,varunarya10/oslo.config,tianshan/oslo.config