add more info

README.md

@@ -129,6 +129,18 @@ safely use `jq` to parse it.
 
 the rest is normal
 
+### cache invalidation
+
+due to how blag works, the hash of the query params is stored alongside the
+mtime of the post file. you can only tell bingus-blog "hey, don't cache me!"
+from a post, but sometimes you need to invalidate the existing cache for all
+query param combinations. to do that, `touch -c "$1"` from your post file.
+this sets the mtime for the post to right now, invalidating the reference to
+it as bingus-blog believes that the contents have changed.
+
+to improve cache availability, it is considered good practice to always set
+`DONT_CACHE` if there is at least one query param.
+
 ### fastblag
 
 :eyes: