Add warning to secret_key parameter.

src/cachelib/base.py

@@ -29,7 +29,10 @@ class BaseCache:
                             no timeout is specified on :meth:`set`. A timeout
                             of 0 indicates that the cache never expires.
 
-    :param secret_key:      Key to sign cache entries with.
+    :param secret_key: Key to sign cache entries with.
+
+        .. versionadded:: 0.3.0
+
     """
 
     def __init__(

src/cachelib/file.py

@@ -27,6 +27,13 @@ class FileSystemCache(BaseCache):
     :param mode: the file mode wanted for the cache files, default 0600
 
     :param secret_key: Key to sign cache entries with.
+
+        .. warning::
+            Without a secret key or in case the secret key is not secret anymore,
+            anyone with write access to the cache directory can trick your program
+            into executing arbitrary code.
+
+        .. versionadded:: 0.3.0
     """
 
     #: used for temporary files by the FileSystemCache

src/cachelib/redis.py

@@ -24,6 +24,13 @@ class RedisCache(BaseCache):
     :param key_prefix: A prefix that should be added to all keys.
     :param secret_key: Key to sign cache entries with.
 
+        .. warning::
+            Without a secret key or in case the secret key is not secret anymore,
+            anyone with write access to the redis instance can trick your program
+            into executing arbitrary code.
+
+        .. versionadded:: 0.3.0
+
     Any additional keyword arguments will be passed to ``redis.Redis``.
     """
 

src/cachelib/uwsgi.py

@@ -18,6 +18,13 @@ class UWSGICache(BaseCache):
         same instance as the werkzeug app, you only have to provide the name of
         the cache.
     :param secret_key: Key to sign cache entries with.
+
+        .. warning::
+            Without a secret key or in case the secret key is not secret anymore,
+            anyone with write access to the uWSGI cache can trick your program
+            into executing arbitrary code.
+
+        .. versionadded:: 0.3.0
     """
 
     def __init__(