Added a simple README file for unit test.

tests/unit/README.md

@@ -0,0 +1,7 @@
+# Unit test
+
+Here in this folder is a set of tests for the Haxe compiler. To compile and run the tests:
+
+ 1. `haxe compile.hxml`
+ 2. `nekotools server`
+ 3. open `http://localhost:2000/unit.html` in your browser