Documentation is missing pygit2 install from repository checkout instructions

[whoopsmith]

The Advanced installation instructions don't provide any guidance on installing pygit2 from a repository checkout.

There are instructions for installing libgit2 but then for pygit2 it only says the following:

Now install pygit2, and then verify it is correctly installed: $ pip install pygit2

This will install the latest released version but not a version checked out from the repository.

Looking at what is provided in the Makefile the following is a reasonable guess and seems to work

cd pygit2 sudo python install

or in my case sudo python3 install.

It would be helpful if the documentation contained some verification that this is recommended method when installing from a repository checkout.