Better explanation of xvfb in README.md file

- xvfb must be installed first
- --no-xvfb argument for development environments

Signed-off-by: Joseph Nuthalapati <njoseph@thoughtworks.com>

Author: JosephNuthalapati2244

README.md

@@ -5,6 +5,7 @@ $ sudo apt install python3-pytest
 $ pip3 install splinter
 $ pip3 install pytest-splinter
 $ pip3 install pytest-bdd
+$ sudo apt install xvfb  # optional, to avoid opening browser windows
 $ pip3 install pytest-xvfb  # optional, to avoid opening browser windows
 ```
 
@@ -49,3 +50,10 @@ can also specify which tests to run, by tag or keyword:
 ```
 $ py.test -k essential
 ```
+
+If xvfb is installed and you still want to see browser windows, use the 
+`--no-xvfb` command-line argument.
+
+```
+$ py.test --no-xvfb -k mediawiki
+```