add a note about ubuntu only runner for now

Author: craig-day

README.md

@@ -9,6 +9,8 @@ This will add the following executables to your `PATH`, making them available fo
 - `dhall-to-yaml`
 - `json-to-dhall`
 
+**Note:** Currently this only supports the `ubuntu-` runtimes for actions. See [TODO](#todo) below.
+
 ### Inputs
 
 #### `version`
@@ -41,3 +43,9 @@ jobs:
       - run: dhall version
       - run: dhall-to-json --version
 ```
+
+### TODO
+
+- [ ] Add platform validation on action to fail early if a non-ubuntu runner is used
+- [ ] Add support for the macOS runner
+- [ ] Add support for the Windows runner