Mention updating `pip` in installation instructions

Created by: PhilipVinc

@gcarleo recently noticed that with pip==19.xxx installing netket fails in a clean py3.7 environment. Looking into it, I found out that pip<=20.3 completely ignored upper version bounds on dependencies.

This madness breaks several packages, in particular numba which upper bounds numpy and flax/optax who upper bound typing_extensions on py3.7.

now the installation instructions say to update pip first. By the way, this is the same thing that jax says in its installation instructions, although for different reasons. Since they are a dependency, we should do the same.