fsthttp: add new caching infrastructure

Author: dgryski

Diff for: CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Added
 
+- fsthttp: add http cache API support
 - secretstore: add Plaintext toplevel convenience function
 - acl: add ACL hostcalls
 

Diff for: README.md

@@ -74,6 +74,15 @@ Logging can be done using a Fastly Compute Log Endpoint ([example](./_examples/l
 fmt.Printf("request received: %s\n", r.URL.String())
 ```
 
+## Readthrough HTTP Cache Support
+
+Customizing cache behaviour with the readthrough cache is an opt-in feature; enable it by adding `-tags=fsthttp_guest_cache` to the build line of your `fastly.toml`.
+
+```
+[scripts]
+build = "tinygo build -target=wasip1 -tags=fsthttp_guest_cache -o bin/main.wasm ."
+```
+
 ## TinyGo Recommended Packages
 
 TinyGo is still a new project, which has yet to get a version `1.0.0`. Therefore, the project is incomplete, but in its current state can still handle a lot of tasks on Compute. However, [some languages features of Go are still missing](https://tinygo.org/docs/reference/lang-support/).