Document run-time deps

README.md

@@ -34,6 +34,28 @@ And install the wheel in `%install` with `%pyproject_install`:
     %pyproject_install
 
 
+Adding run-time and test-time dependencies
+------------------------------------------
+
+To run tests in the `%check` section, the package's runtime dependencies
+often need to also be included as build requirements.
+If the project's build system supports the [`prepare-metadata-for-build-wheel`
+hook](https://www.python.org/dev/peps/pep-0517/#prepare-metadata-for-build-wheel),
+this can be done using the `-r` flag:
+
+    %generate_buildrequires
+    %pyproject_buildrequires -r
+
+For projects that specify test requirements using an [`extra`
+provide](https://packaging.python.org/specifications/core-metadata/#provides-extra-multiple-use),
+these can be added using the `-x` flag.
+For example, if upstream suggests installing test dependencies with
+`pip install mypackage[testing]`, the test deps would be generated by:
+
+    %generate_buildrequires
+    %pyproject_buildrequires -r -x testing
+
+
 Limitations
 -----------
 
@@ -49,6 +71,7 @@ Extras are currently ignored.
 
 Some valid Python version specifiers are not supported.
 
+The `-x` flag does not yet support multiple (comma-separated) extras.
 
 [PEP 517]: https://www.python.org/dev/peps/pep-0517/
 [PEP 518]: https://www.python.org/dev/peps/pep-0518/