Docs for first time

[mslinn]

1. The first time acme-dns-client is run, it must be run as root, or the following error will appear unless the user knew to create /etc/acmedns:

   $ acme-dns-client --help
   Error while starting up: mkdir /etc/acmedns: permission denied
   

   It would be good to generate the help message before the program attempts to access /etc/acmedns. Even better, tell the user that the directory does not exist, which means that acme-dns needs to be configured before using acme-dns-client.

2. Here is a possible addition to the docs: If acme-dns-client is run under sudo, another issue presents. Because sudo filters out environment variables such as GOPATH by default, and changes the default GOPATH to /root/go/bin/, the first time acme-dns-client is run, it must be invoked via its fully qualified path, like this:

   $ sudo /home/user_name/go/bin/acme-dns-client --help