Add platform-specific notes to readme

README.md

@@ -238,6 +238,9 @@ Shell command to run tests after the build. The wheel will be installed automati
 
 Example: `nosetests {project}/tests`
 
+Platform-specific variants also available:
+`CIBW_TEST_COMMAND_MACOS` | `CIBW_TEST_COMMAND_WINDOWS` | `CIBW_TEST_COMMAND_LINUX`
+
 | Environment variable: `CIBW_TEST_REQUIRES`
 | ---
 
@@ -248,6 +251,9 @@ Space-separated list of dependencies required for running the tests.
 Example: `pytest`  
 Example: `nose==1.3.7 moto==0.4.31`
 
+Platform-specific variants also available:
+`CIBW_TEST_REQUIRES_MACOS` | `CIBW_TEST_REQUIRES_WINDOWS` | `CIBW_TEST_REQUIRES_LINUX`
+
 --
 
 #### Example YML syntax