Update documentation.

The current instructions may confuse a new user. The README says to directly clone the repo into the applets folder, however this doesn't allow out of the box support. The user may not understand that they were supposed to "make" first.

The Add Applets program also displays Feeds  as an option to before it's even compiled. When the user chooses to install the applet like that, the program won't actually work, but will be listed as installed. Even if the user then proceeds to "make" after this, Cinnamon will not traverse deeper into the BUILD folder to find the correct software. The user is now stuck until they move the contents of BUILD up to the applets folder, while deleting the Git repo to stop confusing Cinnamon. 

The new instructions will help someone who may be confused. Also, it may not be obvious that compiling will only take the changes committed to Git, so I have added some info about it to prevent a user wondering why their uncommitted changes aren't compiling.