Update /vendor/README.md

since the vendor already stable since 1.5, update the doc to remove the note on vendor still experiment.

vendor/README.md

@@ -1,6 +1,6 @@
 # `/vendor`
 
-Application dependencies (managed manually or by your favorite dependency management tool like the new built-in, but still experimental, [`modules`](https://github.com/golang/go/wiki/Modules) feature).
+Application dependencies (managed manually or by your favorite dependency management tool like the new built-in).
 
 Don't commit your application dependencies if you are building a library.