GitHub updates and new repos

Hi all,

I have migrated the Payment Method Identifiers and Card Payment Method
specs to their own repos as well as migrating issues and updating links
(editors will need to accept my PRs for the spec changes before these will
be public).

All of the WG specs are now indexed from the README on our core repo.
You can go to https://github.com/w3c/webpayments and scroll down for the
list.

This means we have a separate issue list for each spec which means you do
need to jump around to track all the issues but the lists will be cleaner
and more focused.

I will re-look at getting us a dashboard that consolidates all of these
lists.

I am not sure what the group wants to do with the various architecture
specs/proposals. We have an editor's draft in the browser API repo which
the group chose not to advance to FPWD and another proposed spec in the
proposals folder.

My suggestion is to create an w3c/webpayments-architecture repo and put the
current ED in there. The other document would then be proposed as a
replacement for that document.

Adrian