cloning pFUnit with submodules requires git version > 1.8.3.1

[kurtsansom]

Update the documentation to add requirement for newer version of git. i.e. git 1.8.3.1 and git 1.X probably won't work.

git clone --recursive fails for older git versions. other variants of git clone + git submodules --init --recursive also failed.

The issue was resolved by using a newer version of git, version 2.17.1 and 2.30.0 but is not indicated in the documentation.

[kurtsansom]

it appears to be happening on newer versions of git as well. e.g. git version 2.1.4