added missing parts in INSTALL.txt about smarty and addons

INSTALL.txt

@@ -70,6 +70,24 @@ OR
 	cd [web server folder]
 	php bin/composer.phar install
 
+Make sure the folder view/smarty3 exists and is writable by the webserver
+user, in this case `www-data`
+
+    mkdir view/smarty3
+    chown www-data:www-data view/smarty3
+    chmod 775 view/smarty3
+
+Get the addons by going into your website folder.
+
+    cd mywebsite
+
+Clone the addon repository (separately):
+
+    git clone https://github.com/friendica/friendica-addons.git addon
+
+If you copy the directory tree to your webserver, make sure that you also 
+copy .htaccess - as "dot" files are often hidden and aren't normally copied.
+
 3. Create an empty database and note the access details (hostname, username,
 password, database name).