Add building instructions

[stefnotch]

Issue description:

It'd be neat to add "building instructions" in a file like BUILDING.md

Here are mine

1. Install the FlaxEngine Tools for Visual Studio
2. Clone the FlaxAPI
3. Open the FlaxAPI.sln file using Visual Studio and edit the FlaxEngine Project Properties as follows:
   
4. Click on Attach to Flax. This starts Flax or attaches to the current Flax instance.