Need documentation on the steps to add a YANG Model

[arista-hpandya]

The current YANG Model Guidelines doc does not list all the steps required to add a new yang model. For example the unit test structure and where to add them, the sample config db file that needs to be modified, the documentation that needs to be updated to reflect the new/updated DB.

Adding details on these sub-tasks will help devs to conform to a guideline that can be reused and refined to make sure we all follow the best practices when adding our YANG models. The best place to add this would be in doc/mgmt/SONiC_YANG_Model_Guidelines.md

[Shubhf]

Hey @arista-hpandya Can I take up this model?