Add readme, license to repo. Make packer executable.

[ghost]

This adds a README.md file based on the description in the man page and a LICENSE.txt file. It also sets the executable bit on packer.

[mloskot]

I was thinking about similar pull request, then I noticed this one which is fairly dated.
I wonder, is @bruenig no longer mainitaining the packer?

[bruenig]

I am still here. This is mainly just used in arch linux, so the full text of the license is already included. I don't know what the readme does, but we've got a man page. The thing is +x'd by the PKGBUILD that builds it for arch. So basically, all this stuff is already done internally within arch's package system. Is anyone using it outside that system? And if so, why?

[mloskot]

@bruenig Good points. Though, I think README.md in Git still makes sense. Ideally, if the whole man page could be markdown'ed in the README.md and then automatically converted to man page.

[mloskot]

Also, README.md could mention requirements like pacman -S jshon