make readme look nice

Author: chrishalcrow

README.md

@@ -200,21 +200,25 @@ You can create your own custom layout by specifying which views you'd like
 to see, and where they go. The basic window layout supports eight "zones",
 which are laid out as follows:
 
+```
 +---------------+--------------+
 | zone1   zone2 | zone3  zone4 |
 +               +              +
 | zone5   zone6 | zone7  zone8 |
 +---------------+--------------+
+```
 
 If a zone has free space below it or to the right of it, it will try to use it.
 Stretching downwards takes precedence over stretching rightwards.
 E.g. suppose your layout is only non-empty in zones 1, 4, 5, 6 and 7:
 
+```
 +---------------+--------------+
 | zone1         |        zone4 |
 +               +              +
 | zone5   zone6 | zone7        |
 +---------------+--------------+
+```
 
 Then zone1 will stretch right-wards to make a three-zone view. Zone4 will stretch
 downwards to make a long two-zone view.