Add link to DockerCompletion for PowerShell

Signed-off-by: Masatoshi Higuchi <matt9ucci@gmail.com>

contrib/completion/powershell/readme.txt

@@ -1 +1,2 @@
-See https://github.com/samneirinck/posh-docker
+See https://github.com/matt9ucci/DockerCompletion
+or https://github.com/samneirinck/posh-docker