Add a section about the pytest plugin in the README

As we know that many users will use the profiler with the pytest plugin,
add directly a section in the README about it explaining how to use it
and a couple of simple examples.

README.md

@@ -61,7 +61,7 @@ Check your package manager on how to install these dependencies (for example `ap
 
 Once you have the binary dependencies installed, you can clone the repository and follow with the normal building process:
 
-```python
+```shell
 git clone git@github.com:bloomberg/memray.git memray
 cd memray
 python3 -m venv ../memray-env/  # just an example, put this wherever you want
@@ -141,6 +141,52 @@ This will produce an HTML file with a flame graph of the memory usage that you c
 
 <img src="https://github.com/bloomberg/memray/blob/main/docs/_static/images/flamegraph_example.png?raw=true" align="center"/>
 
+## Pytest plugin
+
+If you want an easy and convenient way to use `memray` in your test suite, you can consider using [pytest-memray](https://github.com/bloomberg/pytest-memray). Once installed, this pytest plugin allows you to simply add `--memray` to the command line invocation:
+
+```shell
+pytest --memray tests/
+```
+
+And you can get automatically a report like this:
+
+```
+python3 -m pytest tests --memray
+=============================================================================================================================== test session starts ================================================================================================================================
+platform linux -- Python 3.8.10, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
+rootdir: /mypackage, configfile: pytest.ini
+plugins: cov-2.12.0, memray-0.1.0
+collected 21 items
+
+tests/test_package.py .....................                                                                                                                                                                                                                      [100%]
+
+
+================================================================================================================================= MEMRAY REPORT ==================================================================================================================================
+Allocations results for tests/test_package.py::some_test_that_allocates
+
+	 📦 Total memory allocated: 24.4MiB
+	 📏 Total allocations: 33929
+	 📊 Histogram of allocation sizes: |▂   █    |
+	 🥇 Biggest allocating functions:
+		- parse:/opt/bb/lib/python3.8/ast.py:47 -> 3.0MiB
+		- parse:/opt/bb/lib/python3.8/ast.py:47 -> 2.3MiB
+		- _visit:/opt/bb/lib/python3.8/site-packages/astroid/transforms.py:62 -> 576.0KiB
+		- parse:/opt/bb/lib/python3.8/ast.py:47 -> 517.6KiB
+		- __init__:/opt/bb/lib/python3.8/site-packages/astroid/node_classes.py:1353 -> 512.0KiB
+```
+
+You can also use some of the included markers to cause some selected tests to
+fail if the execution of said test allocates more memory than allowed:
+
+```python
+@pytest.mark.limit_memory("24 MB")
+def test_foobar():
+    # do some stuff that allocates memory
+```
+
+To learn more on how the plugin can be used and configured check out [the plugin documentation](https://bloomberg.github.io/pytest-memray/#).
+
 # Native mode
 
 Memray supports tracking native C/C++ functions as well as Python functions. This can be especially useful when profiling applications that have C extensions (such as `numpy` or `pandas`) as this gives holistic vision of how much memory is allocated by the extension and how much is allocated by Python itself.