Update README.md

Add platform requirements for this library and explicitly state which
Python version it supports, which are both frequent questions in issues
opened for this respository.

Author: exodrifter

README.md

@@ -1,12 +1,39 @@
 UnityPython
 ===========
 UnityPython is a plugin for Unity3D that provides support for running Python
-code in Unity.
+2.x code in Unity3D on any platform which supports `System.Reflection.Emit`.
 
 Special thanks to the developers of IronPython who developed the open-source
 integration of Python and .NET, which this plugin uses.
 
 
+Requirements
+============
+The build target must support `System.Reflection.Emit`. In the Unity3D docs, you
+can find an up-to-date detailed list of which platforms support `Emit` or not
+[here](https://docs.unity3d.com/Manual/ScriptingRestrictions.html).
+
+As of Unity3D 2019.4 LTS, here is a chart of platform support for `Emit`:
+| Platform (scripting backend)        | Is `System.Reflection.Emit` supported? |
+|-------------------------------------|----------------------------------------|
+| Android (IL2CPP)                    | No                                     |
+| Android (Mono)                      | Yes                                    |
+| iOS (IL2CPP)                        | No                                     |
+| PlayStation 4 (IL2CPP)              | No                                     |
+| PlayStation Vita (IL2CPP)           | No                                     |
+| Standalone (IL2CPP)                 | No                                     |
+| Standalone (Mono)                   | Yes                                    |
+| Switch (IL2CPP)                     | No                                     |
+| Universal Windows Platform (IL2CPP) | No                                     |
+| Universal Windows Platform (.NET)   | Maybe*                                 |
+| WebGL (IL2CPP)                      | No                                     |
+| WiiU (Mono)                         | Yes                                    |
+| XBox One (IL2CPP)                   | No                                     |
+
+\* Unconfirmed, as Unity states that UWP (.NET) uses a ".NET Core class
+   libraries subset" but fails to state what libraries are included in that
+   subset. If you know the answer to this question, please open a PR.
+
 Setup
 =====
 Place the source of this project anywhere in the "Assets" folder. For