docs: add comments in forwarder and todo

TODO.md

@@ -1,5 +1,6 @@
 # TODO List
 
+- [Try in v0.5.1 or 0.6.0] Fix strategy for `h2c` requests on forwarded requests upstream. This needs to update forwarder definition. Also, maybe forwarder would have a cache corresponding to the following task.
 - [Try in v0.6.0] **Cache option for the response with `Cache-Control: public` header directive ([#55](https://github.com/junkurihara/rust-rpxy/issues/55))**
 - Improvement of path matcher
 - More flexible option for rewriting path

rpxy-lib/src/handler/forwarder.rs

@@ -23,6 +23,7 @@ where
 {
   // TODO: need `h2c` or http/2-only client separately
   inner: Client<C, B>,
+  // TODO: maybe this forwarder definition is suitable for cache handling.
 }
 
 #[async_trait]