Explain that detached option doesn't block current process also. (#459)

* Recommend using `@async notebook`

* Update README.md

Explain that the detached option does not block the current process

README.md

@@ -69,6 +69,16 @@ notebook()
 to launch the IJulia notebook in your browser.  You can
 use `notebook(detached=true)` to launch a notebook server
 in the background that will persist even when you quit Julia.
+This is also useful if you want to keep using the current Julia
+session instead of opening a new one.
+
+```julia
+julia> using IJulia; notebook(detached=true)
+Process(`'C:\Users\JuliaUser\.julia\v0.4\Conda\deps\usr\Scripts\jupyter' notebook`, ProcessRunning)
+
+julia> 
+```
+
 By default, the notebook "dashboard" opens in your
 home directory (`homedir()`), but you can open the dashboard
 in a different directory with `notebook(dir="/some/path")`.