add instructions to README

README.rst

@@ -78,6 +78,36 @@ The ``asv preview`` command will give the URL of the local web server (e.g.
 http://127.0.0.1:21331) - go to this address in your favorite browser to see
 the results.
 
+Running benchmarks with specific versions of dependencies
+---------------------------------------------------------
+
+If you would like to run the benchmarks against specific version(s) of an
+installed dependency or dependencies, add the ``matrix.req`` entry to 
+``asv.conf.json``, including the version(s) you wish to run benchmarks against:
+
+.. code-block:: diff
+
+    {
+      "version": 1,
+      "project": "astropy",
+      "project_url": "http://www.astropy.org/",
+      "repo": "https://github.com/astropy/astropy.git",
+      "install_command": [
+        "pip install . matplotlib scipy"
+      ],
+    + "matrix": {
+    +   "req": [
+    +       "matplotlib==3.5.1",
+    +       "numpy==1.26.0",
+    +       "numpy==2.0.0rc1"
+    +   ]
+    + },
+      "branches": ["main"],
+      "show_commit_url": "http://github.com/astropy/astropy/commit/",
+      "pythons": ["3.11"],
+      "environment_type": "virtualenv"
+    }
+
 Writing a benchmark
 -------------------