Merge pull request #407 from psibi/readme-fix

Include instruction for installing package name

Author: snoyberg

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 do is `stack install <package-name>`.
+
 Run `stack` for a complete list of commands.
 
 #### Architecture