Improve README: add special note and section about environment variab…

…les in Usage section!

README.mkd

@@ -60,19 +60,27 @@ you can use a project gemset. A project gemset has a '.' ("dot") as the first ch
 
     $ echo '.gems' > .rbenv-gemsets
 
-This will then install your gems into `project/.gems/`. If you don't want to use a "dot directory" to house your gems (but still want to use a project gemset),
+Your gems will then get installed in `project/.gems`.
+If you don't want to use a "dot directory" to house your gems (but still want to use a project gemset!),
 then do something like this instead:
 
     $ echo './gems' > .rbenv-gemsets
 
-This will then install your gems into `project/gems/`.
+Your gems will then get installed in `project/gems`.
 
 The gemset plugin will cause rbenv to search for the `.rbenv-gemsets` file in
 the current directory, as well as in all ancestor directories, so as long
 as there is an `.rbenv-gemsets` file at or above the current directory.
 
-Alternatively, you can set the RBENV_GEMSET_FILE environment variable to the
-absolute path of the gemset file.
+
+*Special environment variable tips and tricks*
+
+`RBENV_GEMSETS`: for a quick install into a certain gemset, you can use RBENV_GEMSETS:
+
+	$ RBENV_GEMSETS=global gem install rake
+
+`RBENV_GEMSET_FILE`: This isn't exactly recommended, but you can set the RBENV_GEMSET_FILE environment variable to the
+absolute path of a gemset file if the gemset file you want isn't found in any of your ancestor directories. You'd better know what you're doing!