Meta-issues for the group

There have been some discussion about how we handle "meta-issues" or
"high-level philosophical questions" related to our work.

The issues list at https://github.com/w3c/webpayments/issues is explicitly
for this purpose.

The GitHub process at W3C is to create a new repo for each spec so when the
time comes for us to produce our first public working draft of a spec a new
dedicated repo will be setup for this purpose.

Until then, the various proposed specs already have dedicated repos and
issue lists where issues relevant to those specs can be raised.

There are cases where the proposals serve as useful examples of the issue
under discussion and in this case it is helpful to cross-reference an issue
from the spec repo or provide excerpts from the proposal in the issue.

I will be making some changes to the issue labels and milestones we use to
better align with the practices being adopted by other groups and will send
out some guidance on this in time.