Automatic builds of MNX specification

(This is sort of GitHub administrivia, but if you are a current or
potential contributor to the spec you may care about it.)

We've just changed the setup of the mnx repository so that the file
`specification/index.html` is automatically built from its Bikeshed source
file `index.bs` whenever a new commit is pushed to the master branch.

The auto-built specification/index.html file is created only on the
gh-pages branch, not on master, and the MNX website
https://w3c.github.io/mnx/ is now serving gh-pages. When you check out
master, you will not see specification/index.html. You'll need to build it
yourself (as you would anyway if you plan to change it).

Along with this change, pull requests will now be automatically checked to
make sure that Bikeshed is happy with the spec and runs without errors. If
it doesn't, GitHub will complain and will discourage you from merging your
change.

Cheers,

.            .       .    .  . ...Joe
Joe Berkovitz