Add twine check and readthedocs reminder to HOW_TO_RELEASE (pydata#3738)

HOW_TO_RELEASE.md

@@ -1,4 +1,4 @@
-How to issue an xarray release in 14 easy steps
+How to issue an xarray release in 16 easy steps
 
 Time required: about an hour.
 
@@ -20,32 +20,37 @@ Time required: about an hour.
       ```
       pytest
       ```
- 4. On the master branch, commit the release in git:
+ 4. Check that the ReadTheDocs build is passing.
+ 5. On the master branch, commit the release in git:
       ```
       git commit -a -m 'Release v0.X.Y'
       ```
- 5. Tag the release:
+ 6. Tag the release:
       ```
       git tag -a v0.X.Y -m 'v0.X.Y'
       ```
- 6. Build source and binary wheels for pypi:
+ 7. Build source and binary wheels for pypi:
       ```
       git clean -xdf  # this deletes all uncommited changes!
       python setup.py bdist_wheel sdist
       ```
- 7. Use twine to register and upload the release on pypi. Be careful, you can't
+ 8. Use twine to check the package build:
+      ```
+      twine check dist/xarray-0.X.Y*
+      ```
+ 9. Use twine to register and upload the release on pypi. Be careful, you can't
     take this back!
       ```
       twine upload dist/xarray-0.X.Y*
       ```
     You will need to be listed as a package owner at
     https://pypi.python.org/pypi/xarray for this to work.
- 8. Push your changes to master:
+10. Push your changes to master:
       ```
       git push upstream master
       git push upstream --tags
       ```
- 9. Update the stable branch (used by ReadTheDocs) and switch back to master:
+11. Update the stable branch (used by ReadTheDocs) and switch back to master:
      ```
       git checkout stable
       git rebase master
@@ -55,20 +60,20 @@ Time required: about an hour.
     It's OK to force push to 'stable' if necessary. (We also update the stable 
     branch with `git cherrypick` for documentation only fixes that apply the 
     current released version.)
-10. Add a section for the next release (v.X.(Y+1)) to doc/whats-new.rst.
-11. Commit your changes and push to master again:
+12. Add a section for the next release (v.X.(Y+1)) to doc/whats-new.rst.
+13. Commit your changes and push to master again:
       ```
       git commit -a -m 'New whatsnew section'
       git push upstream master
       ```
     You're done pushing to master!
-12. Issue the release on GitHub. Click on "Draft a new release" at
+14. Issue the release on GitHub. Click on "Draft a new release" at
     https://github.com/pydata/xarray/releases. Type in the version number, but
     don't bother to describe it -- we maintain that on the docs instead.
-13. Update the docs. Login to https://readthedocs.org/projects/xray/versions/
+15. Update the docs. Login to https://readthedocs.org/projects/xray/versions/
     and switch your new release tag (at the bottom) from "Inactive" to "Active".
     It should now build automatically.
-14. Issue the release announcement! For bug fix releases, I usually only email
+16. Issue the release announcement! For bug fix releases, I usually only email
     [email protected]. For major/feature releases, I will email a broader
     list (no more than once every 3-6 months):
       - [email protected]