Alternate Portfile instructions for Macports

[randywallace]

I've created an alternate Portfile for macports that installs the most recent version of Mosh. I've included instructions on the following site which I believe are pretty easy to follow... this may help mitigate a lot of repeat bug reports in the mosh repo for things that have been fixed in the last year or so: https://github.com/randywallace/mosh-portfile