explain where vignettes are available

README.md

@@ -36,7 +36,7 @@ Now you can install the package with vignettes by using
 ```r
 devtools::install(build_vignettes = TRUE)
 ```
-This creates a `doc/` directory with all built vignettes.
+This makes all vignettes available via `browseVignettes()`.
 
 # Features