Add disabling locksverify to README

[milkey-mouse]

As seen in #4, even successful pushes/pulls often give this warning:

Remote "origin" does not support the Git LFS locking API. Consider disabling it with:
  $ git config lfs.<unknown>.locksverify false

We probably can't ever support the locking API (not without introducing another service—D1?—to store state), so we should mention disabling this in the README. When the page for #5 generates a list of commands to run including setting the URL it should also include one to disable locksverify.