Added install instructions and script

Hey Pester Team,

I created an install script inspired by the posh-git project and updated the readme to instruct how you can install the pester.

Some things to consider (before merging the PR).

1. My instructions about setting powershell to run `Unrestricted` may be mis-leading. If possible would be good to ask people to set their environment to run `RemoteSigned` or `Unrestricted`, but I'm not familiar with remote signed or this project enough to know if that would work for us.

2. Would also be good to include an alternative way of installing via Chocolatey in the instructions, however I think that package didn't work (at least for me). After running `cinst pester` I could not execute any of the pester commands... If that were to get fixed to install itself similar to what is done in this patch's install script then I think it would be a great alternative to include in the install instructions.

Thoughts?