Add an overview of code layout to Contributing guide

CONTRIBUTING.md

@@ -56,3 +56,11 @@ Releases are built and published to pypi based on **git tags.**
 [Milestones](https://github.com/JWCook/aiohttp-client-cache/milestones) will be used to track
 progress on major and minor releases.
 
+
+## Code Layout
+Here is a brief overview of the main classes and modules:
+* `session.CacheMixin`, `session.CachedSession`: A mixin and wrapper class for `aiohttp.ClientSession`, respectively. There is little logic  here except wrapping `_request()` with caching behavior
+* `response.CachedResponse`: A wrapper class built from an `aiohttp.ClientResponse`, with additional cache-related info. This is what is serialized and persisted to the cache.
+* `backends.base.CacheBackend`: Most of the caching logic lives here, including saving and retriving responses, creating cache keys, expiration, etc. It contains two `BaseCache` objects for storing responses and redirects, respectively. By default this is just a non-persistent dict cache.
+* `backends.base.BaseCache`: Base class for lower-level storage operations, overridden by individual backends.
+* Other backend implementations in `backends.*`: A backend implementation subclasses `CacheBackend` (for higher-level operations), as well as `BaseCache` (for lower-level operations).

aiohttp_client_cache/response.py

@@ -42,8 +42,9 @@ def from_object(cls, request_info):
 
 @attr.s(slots=True)
 class CachedResponse:
-    """A dataclass containing cached response information. It will mostly behave the same as a
-    :py:class:`aiohttp.ClientResponse` that has been read, with some additional cache-related info.
+    """A dataclass containing cached response information, used for serialization.
+    It will mostly behave the same as a :py:class:`aiohttp.ClientResponse` that has been read,
+    with some additional cache-related info.
     """
 
     method: str = attr.ib()