add reamde for .pytest_cache

method - `ensure_readme()`

src/_pytest/cacheprovider.py

@@ -103,6 +103,22 @@ class Cache(object):
             with f:
                 self.trace("cache-write %s: %r" % (key, value))
                 json.dump(value, f, indent=2, sort_keys=True)
+                self._ensure_readme()
+
+    def _ensure_readme(self):
+        content_readme = '''# pytest cache directory #
+
+This directory contains data from the pytest's cache plugin, \
+which provides the `--lf` and `--ff` options, as well as the `cache` fixture. 
+
+**Do not** commit this to version control.
+
+See [the docs](https://docs.pytest.org/en/latest/cache.html) for more information.
+        '''
+        if self._cachedir.check(dir=True):
+            readme_path = self._cachedir.join("README.md")
+            if not readme_path.check(file=True):
+                readme_path.write(content_readme)
 
 
 class LFPlugin(object):