Update README for Building instructions.

For ./go to work the chromedriver must be on the devs PATH.
Fixes #1781

Signed-off-by: Luke Inman-Semerau <luke.semerau@gmail.com>

README.md

@@ -42,6 +42,10 @@ see directories carrying multiple build directive files.
 For reference, crazyfun's build files are named *build.desc*,
 while buck's are named simply *BUCK*.
 
+Before building ensure that you have the 
+[most recent `chromedriver` ](https://sites.google.com/a/chromium.org/chromedriver/downloads)
+available on your `$PATH`.  
+
 To build Selenium, in the same directory as this file:
 
 ```sh