Mention -dev packages

Fixes #18

README.markdown

@@ -6,7 +6,7 @@ feature found in Windows XP (and possibly later?)
 ![screenshot.gif](https://raw.githubusercontent.com/arp242/find-cursor/master/screenshot.gif)
 
 Installation
-============
+------------
 
 Compile it by typing `make`, install it with `make install`. There are some
 packages:
@@ -14,14 +14,18 @@ packages:
 - [FreeBSD](https://www.freshports.org/x11/find-cursor/)
 - [Void Linux](https://github.com/void-linux/void-packages/tree/master/srcpkgs/find-cursor)
 
+You'll need to install some X11 header files on some systems; e.g. on
+Ubuntu/Debian: `libx11-dev`, `libxcomposite-dev`, `libxdamage-dev`, and
+`libxrender-dev`.
+
 Usage
-=====
+-----
 
 See `find-cursor(1)` or `find-cursor -h` to see some options for controlling the
 appearance.
 
 Launching
-=========
+---------
 
 You will want to map a key in your window manager to run `find-cursor`. You can
 also use [`xbindkeys`](xbindkeys), which should work with `$any` window manager.
@@ -37,7 +41,7 @@ I don't have a numpad on my keyboard; you can also use `F13` or some other
 unused key.
 
 Compton
-=======
+-------
 
 You may want to disable shadows if you use compton or some other composite
 manager; for example for compton start it with: