Add documentation and examples for scaling to larger screens.

Document all attributes to root element (#impress) in DOCUMENTATION.md.

For the main demo (/index.html) add width, height, max-scale, min-scale
attributes to the main demo. These just use the defaults, except
data-max-scale="3" which allows to scale to larger screens (HD, 4K).

For examples/classic-slides, add attributes to root element and set it
to target Full HD 1920 x 1080 resolution. Adjust CSS to match.

Add notification everywhere that defaults will change in 2021.

index.html

@@ -130,25 +130,40 @@
     That's the wrapper for your presentation steps. In this element all the impress.js magic happens.
     It doesn't have to be a `<div>`. Only `id` is important here as that's how the script find it.
     
-    You probably won't need it now, but there are some configuration options that can be set on this element.
-    
+    Often you don't need to use any attributes here, but for educational purposes I have used all of them below.
+
     To change the duration of the transition between slides use `data-transition-duration="2000"` giving it
     a number of ms. It defaults to 1000 (1s).
     
+    When authoring impress.js presentations, you should target some screen size, which you can define here.
+    The default is 1024 x 768. You should write your CSS as if this is the screen size used for the
+    presentation. When you present your presentation on a screen (or browser window) of different size,
+    impress.js will automatically scale the presentation to fit the screen. The minimum and maximum limits
+    to this scaling can also be defined here.
+    
+    NOTE: I intend to change the defaults to target HD screens in 2021. So you may want to make a habit
+    of explicitly defining these attributes for now, to avoid any disruption when the defaults change.
+    
     You can also control the perspective with `data-perspective="500"` giving it a number of pixels.
     It defaults to 1000. You can set it to 0 if you don't want any 3D effects.
     If you are willing to change this value make sure you understand how CSS perspective works:
     https://developer.mozilla.org/en/CSS/perspective
     
-    But as I said, you won't need it for now, so don't worry - there are some simple but interesting things
-    right around the corner of this tag ;)
+    Plugins:
     
-    ----------
-    Plugins: We set the default time for autoplay plugin to 6 seconds. Autoplay will automatically advance
+    We set the default time for autoplay plugin to 7 seconds. Autoplay will automatically advance
     to next slide after a timeout expires.
-    
 -->
-<div id="impress" data-autoplay="7">
+<div id="impress"
+    data-transition-duration="1000"
+    
+    data-width="1024"
+    data-height="768"
+    data-max-scale="3"
+    data-min-scale="0"
+    data-perspective="1000"
+    
+    data-autoplay="7">
 
     <!--