README.md: clarify OAuth instructions for developer instance
README.md: clarify OAuth instructions for developer instance

BUG=None
TEST=None

Change-Id: I38a0bf43866628d2c14c417a07fc38358444dfb5
Reviewed-on: https://chromium-review.googlesource.com/1148767
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py