Added addon installation instruction. Exclusive repo.

doc/Install.md

@@ -38,7 +38,23 @@ you might have trouble getting everything to work.]
 
         `git clone https://github.com/friendika/Free-Friendika.git mywebsite`
 
-    and then you can pick up the latest changes at any time with
+    - and then you can pick up the latest changes at any time with
+
+        `git pull`
+    
+    - For installing addons
+    
+        - First you should be **on** your website folder
+        
+            `cd mywebsite`
+            
+        - Then you should clone the addon repository (separtely)
+        
+            `git clone https://github.com/friendika/friendika-addons.git addon`
+            
+        - For keeping the addon tree updated, you should be on you addon tree and issue a git pull
+        
+            `cd mywebsite/addon`
             
             `git pull`