Document mono 5.x requirement

Author: kjnilsson

BUILD_DOTNET_CORE.md

@@ -1,6 +1,8 @@
 To build the dotnet core components you need the `dotnet` command line utility installed.
 It can be obtained for your chosen platform [here](https://www.microsoft.com/net/core#macosx).
 
+You will also need [mono 5.x](http://www.mono-project.com/docs/about-mono/releases/).
+
  Run `./build.sh` (linux/osx) or `build.bat` (windows) to trigger code generation and complete a dotnet core build. 
 
  To run the tests simply run `run-tests.sh` (linux/osx) or `run-tests.bat` (windows). This will run the entire test suite. There is currently no way to only run a subset of tests. 

README.md

@@ -33,7 +33,8 @@ See [ChangeLog.md](https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/maste
 
 ## Building from Source
 
-Please see [How to Run Tests](./RUNNING_TESTS.md) for build process overview.
+Please see [How to Run Tests](./RUNNING_TESTS.md) and [Building .NET Core](./BUILD_DOTNET_CORE.md)
+for build process overview.
 
 ## Contributing