Document enable/disable ext options

Author: honno

README.md

@@ -10,6 +10,8 @@ specification that are not yet tested here.
 
 ## Running the tests
 
+### Setup
+
 To run the tests, first install the testing dependencies
 
     pip install pytest hypothesis
@@ -20,6 +22,8 @@ or
 
 as well as the array libraries that you want to test. 
 
+### Specifying the array module
+
 To run the tests, you need to set the array library that is to be tested. There
 are two ways to do this. One way is to set the `ARRAY_API_TESTS_MODULE`
 environment variable. For example
@@ -41,6 +45,29 @@ import numpy as array_module
 
 (replacing `numpy` with the array module namespace to be tested).
 
+### Run pytest
+
+Simply run the `pytest` command to run the whole test suite.
+
+```bash
+pytest
+```
+
+The test suite tries to logically organise its tests so you can find specific
+test cases whilst developing something in particular. So to avoid running the
+rather slow complete suite, you can specify particular test cases like any other
+test suite.
+
+```bash
+pytest array_api_tests/test_creation_functions.py::test_zeros
+```
+
+By default, tests for the optional Array API extensions such as
+[`linalg`](https://data-apis.org/array-api/latest/extensions/linear_algebra_functions.html)
+will be skipped if not present in the specified array module. You can purposely
+skip testing extension(s) via the `--disable-extension` option, and likewise
+purposely test them via the `--enable-extension` option.
+
 ## Notes on Interpreting Errors
 
 - Some tests cannot be run unless other tests pass first. This is because very