Add note on how to run just one module's tests (#1455)

README.md

@@ -61,10 +61,18 @@ Weasyl should now be running at <http://weasyl.localhost:8080/>! Several account
 
 ## Running tests
 
+To run all tests:
+
 ```shell
 ./wzl test
 ```
 
+To run only a specific module's tests, such as `weasyl.test.test_api`:
+
+```shell
+./wzl test pytest -x weasyl.test.test_api
+```
+
 
 ## Making migrations