Add README.md in pkg/apis/istio/v1alpha2/

[akyyy]

/area API
/kind doc

Expected Behavior

We should use vendor's tool to generate Istio objects

Actual Behavior

We crafted our own version for istio types.
https://github.com/elafros/elafros/blob/c2c01208fee15326465ceddcb55174bbdb6c3192/pkg/apis/istio/v1alpha2/routerule_types.go#L34

Additional Info

From @evankanderson:
It looks like this was hand-constructed; looking at https://github.com/istio/api/blob/master/routing/v1alpha1/route_rule.pb.go#L211 (which I presume is the corresponding file, though it says "v1alpha1"), it looks like these are part of Istio.
We should have a README.md in pkg/apis/istio/v1alpha2/ explaining what's going on with these files and how they were constructed. (What was the source inspiration, why they aren't simply vendored, etc.)

[akyyy]

@mattmoor , do you know the right person for this item?

[evankanderson]

I'm suggesting documenting the status quo for now; a better solution would be to replace the current objects with some form of proper dependency import.

[mattmoor]

This is all just a means to the end of generating an Istio client with k8s.io/code-generator. I'd certainly prefer to do what we're doing (now) with Build, but AFAIK they don't check in client code we can simply vendor.

Maybe we can vendor their types and codegen from that, but someone would need to spend some time experimenting with options (this was just the most expedient early on).

[mattmoor]

/assign @mdemirhan
/unassign @mattmoor

I think @mdemirhan probably owns the story for this longer-term, so assigning to him.