Add a note on the different versions.

README.md

@@ -18,6 +18,14 @@ is exported.
 npm install mdi-material-ui --save
 ```
 
+There are three major versions of this package, each one for different Material-UI releases. All three versions will receive icon updates as minor releases for the foreseeable future.
+
+| Material-UI | mdi-material-ui |
+|---|---|
+|0.x|^4.0.0|
+|^1.0.0, ^3.0.0|^5.0.0|
+|^4.0.0|^6.0.0|
+
 ## Usage
 Every icon is exported with its original name in PascalCase. So `coffee` becomes `Coffee`,
 `cloud-print-outline` is exported as `CloudPrintOutline` and so on.