installation documentation suggestion

[ismael-mendoza]

The documentation suggests to install scarlet via

git clone https://github.com/pmelchior/scarlet.git
cd scarlet
python setup.py install

For the last line, I find that at least on my Mac OS the following works better (thanks @beckermr for the suggestion!):

pip install .

otherwise I get some strange c++ missing header errors. I just wanted to suggest to either change the documentation or add this as an additional suggestion for people who are having the same issue.