Install instructions fail on Windows if ".CMD" is not in %PATHEXT%

I had trouble installing keybase after the node.js setup because I didn't have .CMD files set in my PATHEXT. Since the executables are all .cmd files, not having the system recognize that caused `npm` not to be recognized, then when I realized I had to do `npm.cmd install -g keybase-installer`, I still couldn't find `keybase-installer`!

Finding that in `%APPDATA%/npm` didn't help, since running `keybase-installer.cmd` threw an error that it couldn't find `npm` on the system's PATH.

The solution, of course, is simply adding `.CMD` to your pathext, or alternatively running `SET PATHEXT=%PATHEXT%;.CMD` in your shell before running the installation steps.

Review the Windows installation steps and the npm/keybase-installer commands described in the report, starting by reproducing them with .CMD absent from PATHEXT. Update the installation guidance so the commands work in that environment, then verify that keybase-installer can be found and run successfully.

Written by the indexing model from the issue text.