README.auth_keys

* generate personal access token for github (https://github.com/settings/tokens/new)
* for .travis.yml, encrypt it using "travis encrypt", e.g.
   travis encrypt -r MacPython/cftime-wheels --org GITHUB_API_TOKEN="<github personal token>"
   env:
      - secure: <generated string>
   deploy:
     provider: releases
     api_key: $GITHUB_API_TOKEN
* for appveyor.yml, copy and paste it into https://ci.appveyor.com/tools/encrypt
   artifacts:
     - path: /.*\.whl/
   deploy:
     - provider: GitHub
       description: "cftime release"
       auth_token: 
         secure: <generated string>
       force_update: true
       on:
         APPVEYOR_REPO_TAG: true 
* to delete and re-create a tag (to trigger re-uploading new wheels)
   git tag -d vX.Y.Zrel
   git push --delete origin vX.Y.Zrel
   git tag -a vX.Y.Zrel -m "version X.Y.Z release"
   git push origin --tags