aruco make / install instructions incorrect

[fnoop]

git clone https://github.com/fnoop/aruco
git checkout 3.1.0
cd aruco
cmake . && make && sudo make install

cd aruco should be before the checkout, and there's no 3.1.0 branch.
Current master is 3.0.11 which compiles fine on opencv 3.2, but probably not on 4.x
3.0.13 is the last opensource version, but doesn't compile on opencv 3.2 (does compile on 4.x).

[fnoop]

install doesn't work:

~/aruco$ sudo cmake install
CMake Error: The source directory "/home/fnoop/aruco/install" does not exist.
Specify --help for usage, or press the help button on the CMake GUI.

[fnoop]

git clone https://github.com/fnoop/aruco
cd aruco
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr/local/aruco -DCMAKE_INSTALL_RPATH=/usr/local/aruco/lib
make && sudo make install

[fnoop]

python2 no longer installed by default on ubuntu 18.04.
sudo apt -y install python python-pip