Add a page for building for mono

Building-Testing-and-Debugging-on-Mono-MSBuild.md

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+MSBuild can be successfully built on Windows, OS X 10.11, Ubuntu 14.04, and Ubuntu 16.04.
+
+Mono maintains a fork of msbuild (for now) at `https://github.com/mono/msbuild/`. You can clone that and use the `xplat-master` branch or `d15.3` for the next release branch.
+
+# Unix #
+
+**Required packages for OSX & Ubuntu**
+
+MSBuild currently builds with a prerelease version of .NET Core 1.0. It requires the [.NET Core prerequisites](https://github.com/dotnet/core/blob/master/Documentation/prereqs.md), which you can acquire manually or easily get by [installing the .NET Core SDK](https://dot.net/core).
+
+* *OpenSSL*: MSBuild uses the .Net CLI to download Nuget packages during its build process. The CLI requires a recent OpenSSL library available in `/usr/lib`. This can be downloaded using [brew](http://brew.sh/) on OS X (`brew install openssl`) and apt-get (`apt-get install openssl`) on Ubuntu, or [building from source](https://wiki.openssl.org/index.php/Compilation_and_Installation#Mac). If you use a different package manager and see an error that says `Unable to load DLL 'System.Security.Cryptography.Native'`, `dotnet` may be looking in the wrong place for the library.
+
+* [Mono](http://www.mono-project.com/download/) when doing a Mono-hosted version of MSBuild
+
+**Required packages for Ubuntu**
+* [libunwind](http://www.nongnu.org/libunwind/index.html) is required by .NET Core. Install it using `sudo apt-get install libunwind8`
+
+## Build process ##
+
+```make```
+
+## Tests ##
+
+```make test-mono```
+
+## Installing ##
+
+`./install-mono-prefix.sh </your/mono/prefix>`
+
+## Getting Mono MSBuild binaries without building the code ##
+The best way to get Mono MSBuild for OSX/macOS is to get the official [Mono package](http://www.mono-project.com/download/#download-mac). After installing it, you can run `msbuild`.
+<br/>
+For Linux, you can install mono and msbuild from [here](http://www.mono-project.com/download/#download-lin).