Improve the introduction's expression.

Author: Ace-Who

README.md

@@ -1,13 +1,16 @@
 # vim-fontsize
 
-Change font size easily.
+Change font size incrementally by one-key typing.
 
 Currently only works on Windows and Mac OS X (not tested).
 
 ## Usage
 
 1. Run `:Fontsize` to enter the state of changing font size.
-2. Simply press `+` (or `=`) and `-` to increase and decrease the font size respectively. It changes font size by 0.5 in height or width every time.
+2. Press `+`/`=` and `-` to increase and decrease the font size respectively,
+   by 0.5 in height, if width is not the only dimension found in the 'guifont'
+   setting, or width every time.
 3. Press `<Esc>` to quit the state.
 
-It works by changing option `guifont`. Read `:help 'guifont'` to find if your system uses this option.
+It works by changing option `guifont`. Refer to `:help 'guifont'` to find if
+your system uses this option.