Add pre-reqs doc to the repo (#2219)

* Add pre-reqs doc to the repo 

This document outlines all of the native dependencies that are needed to run the CLI tools for Ubuntu, Windows, CentOS and OS X. 
As per chat with @leecow documentation does not need special approval to get merged. 

skip ci please

* Update cli-prerequisites.md

* Responding to PR feedback

I've changed the Ubuntu reqs to be the same as CentOS pre-reqs just with the correct package names. This seems like a more sane thing to do.

* Update cli-prerequisites.md

Documentation/cli-prerequisites.md

@@ -0,0 +1,50 @@
+CLI native prerequisites
+=========================
+
+This document outlines the dependencies needed to run .NET Core CLI tools. Most of these dependencies are also .NET Core's general dependencies, so installing them will make sure that you can run applications written for .NET Core in general.
+
+## Windows dependencies
+On Windows, the only dependency is the VC++ Redistributable. Depending on the version of Windows you are running on, the versions are changing.
+
+> **Note:** these dependencies are chained in the installer, you only need to install them manually if you are using
+> the installer script (`dotnet-install.sh` or `dotnet-install.ps1`). 
+
+* Windows 10
+    * [Visual C++ Redistributable for Visual Studio 2015](https://www.microsoft.com/en-us/download/details.aspx?id=48145)
+* Pre-Windows 10
+    *  [Visual C++ Redistributable for Visual Studio 2012 Update 4](https://www.microsoft.com/en-us/download/confirmation.aspx?id=30679)
+
+## Ubuntu
+Ubuntu distributions require the following libraries installed:
+
+- libunwind8 
+- libunwind8-dev
+- gettext
+- libicu-dev
+- liblttng-ust-dev
+- libcurl4-openssl-dev
+- libssl-dev
+- uuid-dev
+- unzip
+
+
+## CentOS
+CentOS distributions require the following libraries installed:
+
+* deltarpm
+* epel-release
+* unzip
+* libunwind
+* gettext 
+* libcurl-devel 
+* openssl-devel 
+* zlib 
+* libicu-devel
+
+## OS X 
+OS X requires the following libraries and versions installed:
+
+* libssl 1.1
+
+## Installing the dependencies
+Please follow the recommended practices of each operating system in question. For Linux, we recommend using your package manager such as `apt-get` for Ubuntu and `yum` for CentOS. For OS X and upgrading the libssl, we recommend using [Homebrew](https://brew.sh/); if you do use it, do not forget the link phase you need to do at the end of install (`brew link openssl --force`).