Add CNAME to docs directory
Add CNAME to docs directory

Automatic documentation deployment constantly deletes `CNAME` file from
`gh-pages` branch, and solution to this seems to be including the file
in `docs` directory of `master` branch instead.

RauliL/plorth,peelonet/plorth,RauliL/plorth,RauliL/plorth,RauliL/plorth