Update README and add clean in Makefile

Makefile

@@ -4,3 +4,5 @@ all: resources.py
 %.py: %.qrc
 	pyrcc4 -o $@ $<
 
+clean: $(shell git clean -fd)
+

README.md

@@ -20,13 +20,15 @@ The annotation file will be saved as an XML file. The annotation format is PASCA
 Requires at least [Python 2.6](http://www.python.org/getit/) and has been tested with [PyQt
 4.8](http://www.riverbankcomputing.co.uk/software/pyqt/intro).
 
-In order to build the resource and assets, you need to install pyqt4-dev-tools:
+In order to build the resource and assets, you need to install pyqt4-dev-tools and lxml:
+```
+$ sudo apt-get install pyqt4-dev-tools
+$ sudo pip install lxml
+$ make all
+$ ./labelImg.py
+```
 
-`$ sudo apt-get install pyqt4-dev-tools`
-
-`$ make all`
-
-`$ ./labelImg.py`
+Mac requires "$ brew install libxml2" when installing lxml
 
 * Windows
 
@@ -34,9 +36,10 @@ Need to download and setup [Python 2.6](https://www.python.org/downloads/windows
 
 Open cmd and go to [labelImg]
 
-`$ pyrcc4 -o resources.py resources.qrc`
-
-`$ python labelImg.py`
+```
+$ pyrcc4 -o resources.py resources.qrc
+$ python labelImg.py
+```
 
 ## Usage
 After cloning the code, you should run `$ make all` to generate the resource file.