Updated instructions to include pre-requisites

I included the instructions but the installation still failed for me .. 

I'm getting: 
```bash
➜  maim git:(master) make && sudo make install            
g++  -O2 -g   -c -o main.o main.cpp
main.cpp: In function ‘int main(int, char**)’:
main.cpp:190:100: warning: embedded ‘\0’ in format [-Wformat-contains-nul]
         sprintf( temp, "%ix%i+%i+%i\0", options.w_arg, options.h_arg, options.x_arg, options.y_arg );
                                                                                                    ^
g++  -O2 -g   -c -o x.o x.cpp
cc    -c -o cmdline.o cmdline.c
g++  -O2 -g   -c -o im.o im.cpp
g++ main.o x.o cmdline.o im.o -o maim -lImlib2 -lX11 -lXfixes -lXrandr
mkdir -p "/usr/bin"
cp /home/rock/projects/maim/maim "/usr/bin"
```