docs: Add an example for EGL_DRIVERS_PATH.

EGL_DRIVERS_PATH can be set to test EGL without installation.
diff --git a/docs/egl.html b/docs/egl.html
index ff610c3..33e9187 100644
--- a/docs/egl.html
+++ b/docs/egl.html
@@ -134,6 +134,16 @@
 addition to the default directory.  This variable is ignored for setuid/setgid
 binaries.</p>
 
+<p>This variable is usually set to test an uninstalled build.  For example, one
+may set</p>
+
+<pre>
+  $ export LD_LIBRARY_PATH=$mesa/lib
+  $ export EGL_DRIVERS_PATH=$mesa/lib/egl
+</pre>
+
+<p>to test a build without installation</p>
+
 </li>
 
 <li><code>EGL_DRIVER</code>