make readme more detailed

README.md

@@ -4,8 +4,10 @@
 [![Go](https://github.com/janos/singleflight/workflows/Go/badge.svg)](https://github.com/janos/singleflight/actions?query=workflow%3AGo)
 
 Package singleflight provides a duplicate function call suppression
-mechanism similar to golang.org/x/sync/singleflight but with support
-for context cancelation.
+mechanism similar to [golang.org/x/sync/singleflight](https://pkg.go.dev/golang.org/x/sync/singleflight) but with support
+for context cancelation. The context passed to the callback function is a context that preserves all values
+from the passed context but is cancelled by the singleflight only when all
+awaiting caller's contexts are cancelled.
 
 ## Installation