Add an initial features doc
Add an initial features doc

Add an overview and initial development guidelines. Restructured
the doc for a better readabiltiy and easier review, and per the
previous review feedback. The TODOs will be addressed iteratively.

Related #13775

Signed-off-by: Sahdev Zala <e752d44c9d22c9ff5ec46b8ef5c7507f41d82165@us.ibm.com>

ajityagaty/etcd,ajityagaty/etcd,ajityagaty/etcd,etcd-io/etcd,etcd-io/etcd,etcd-io/etcd