Update gRIBI `motivation.md` based on evolution. (#35)
Update gRIBI `motivation.md` based on evolution. (#35)

* Update gRIBI `motivation.md` based on evolution.

 * (M) doc/motivation.md
  - Describe motivation for changes to liveliness handling.
  - Describe motivation for selection between clients (leader election)

Co-authored-by: Xiao Wang <5d77bf203d7d491becef05fd6bf97db039ef3081@users.noreply.github.com>
openconfig/gribi