README.md directions when Chocolatey install fails with 407

Chocolatey installation can fail behind a proxy requiring authentication due to [HTTP 407](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.8). This change provides an additional command line invocation for those cases that uses the default credentials for proxy authentication prior to downloading.
Merge pull request #842 from atifaziz/patch-1

README.md directions when Chocolatey install fails with 407