comments in both readme and ini file + repair travis

.travis.yml

   - "3.5"
   - "3.6"
 
+addons:
+  apt:
+    packages:
+    - libreoffice
+
 services:
   - docker
   - redis-server

README.md

     sudo apt install git
     sudo apt install python3 python3-venv python3-dev python3-pip
     sudo apt install redis-server
+    sudo apt install zlib1g-dev libjpeg-dev
+
+for better preview support:
+
+    sudo apt install libreoffice # most office documents file and text format
+    sudo apt install inkscape # for .svg files.
 
 ### Get the source ###
 

development.ini.sample

 ## Do not set http:// prefix.
 # wsgidav.client.base_url = 127.0.0.1:<WSGIDAV_PORT>
 
+### Preview
+## You can parametrized allowed jpg preview dimension list, if not set, default
+## is 256x256. First {width}x{length} items is default preview dimensions.
+## all items should be separated by ',' and you should be really careful to do
+## set anything else than '{int}x{int}' item and ', ' separator
+# preview.jpg.allowed_dims = 256x256,1000x1000
+## Preview dimensions can be set as restricted, if set as restricted, access
+## endpoint to  to get any other preview dimensions than allowed_dims will
+## return error
+# preview.jpg.restricted_dims = True
+
 ###
 # wsgi server configuration
 ###