Include instruction for installing package name

In my opinion, most newbie wants to know if they can [install a package
using `stack`](https://stackoverflow.com/questions/31030438/how-to-install-a-package-using-stack).
And adding that in README is essential IMO.

[ci skip]

Author: psibi

README.md

@@ -43,6 +43,9 @@ You may be prompted to run some of the following along the way:
 * `stack setup` to download and install the correct GHC version. (For
   information on installation paths, please use the `stack path` command.)
 
+If you just want to install a package using stack, then all you have
+to is `stack install <package-name>`.
+
 Run `stack` for a complete list of commands.
 
 #### Architecture