Merge pull request #1249 from asveepay/update_doc

Update install docs for permission denied error

docs/install.md

@@ -23,6 +23,8 @@ To install Compose, run the following commands:
     curl -L https://github.com/docker/compose/releases/download/1.1.0/docker-compose-`uname -s`-`uname -m` > /usr/local/bin/docker-compose
     chmod +x /usr/local/bin/docker-compose
 
+> Note: If you get a "Permission denied" error, your `/usr/local/bin` directory probably isn't writable and you'll need to install Compose as the superuser. Run `sudo -i`, then the two commands above, then `exit`.
+
 Optionally, you can also install [command completion](completion.md) for the
 bash shell.