dpctl installation instruction in readme

[shssf]

(base) :~$ python -m pip install --index-url https://pypi.anaconda.org/intel/simple -extra-index-url https://pypi.org/simple dpctl
ERROR: xtra-index-url is not a valid editable requirement. It should either be a path to a local project or a VCS URL (beginning with bzr+http, bzr+https, bzr+ssh, bzr+sftp, bzr+ftp, bzr+lp, bzr+file, git+http, git+https, git+ssh, git+git, git+file, hg+file, hg+http, hg+https, hg+ssh, hg+static-http, svn+ssh, svn+http, svn+https, svn+svn, svn+file).