Document how unit tests are defined and run

This closes issue shader-slang#4771.

Author: aleino-nv

tools/slang-test/README.md

@@ -56,7 +56,17 @@ The different APIs are
 * DirectD3D11 -dx11,d3d11
 
 
-It may also be necessary to have the working directory the root directory of the slang distribution - in the example above this would be "E:\slang\". 
+It may also be necessary to have the working directory the root directory of the slang distribution - in the example above this would be "E:\slang\".
+
+## Unit tests
+
+In addition to the above test tools, there is also `slang-unit-test-tool`, which is invoked as in the following example.
+
+```
+slang-test slang-unit-test-tool/byteEncode
+```
+
+This will run the `byteEncode` test. The unit tests are located in the https://github.com/shader-slang/slang/tree/master/tools/slang-unit-test directory, and defined with macros like `SLANG_UNIT_TEST(byteEncode)`.
 
 ## Test Categories