[MonoDevelop] Update "Addin Development Basics" with new install path

[Joshua Granick]

Hi!

I am trying out add-in development for MonoDevelop, and enjoy the "Addin  
Development Basics" tutorial, but the install path is wrong.

In fact, it took a lot of searching to find this out. Even the "User  
Profiles" article had the wrong install path.

I'm using MonoDevelop 2.8 on Ubuntu 12.04. The install path is:


  ~/.local/share/MonoDevelop-2.8/LocalInstall/Addins



If the beginning articles could be updated to use the correct install  
path, it would be very helpful. If you create a basic C# library and  
addin.xml, and can't see it in the list of add-ins, it's very frustrating  
and confusing.

Also, on an unrelated note, the passphrase for creating an account for the  
MonoDevelop website is insane! It's good to keep out spam, but it took me  
four tries before I got one right :)