Requirement of amuse-devel requires different installation instructions

[JaroCamphuijsen]

As per commit https://github.com/omuse-geoscience/omuse/commit/6aacba69e9e0d4d2a6c2d8ee840a27f51f39b64a where the dependency on amuse-framework was changed to amuse-develop in setup.py, the installation instructions are out of date.

amuse-develop should be installed by cloning the amuse repository (https://github.com/amusecode/amuse) and following the installation instructions in https://amuse.readthedocs.io/en/latest/install/howto-install-AMUSE.html#development-build

[JaroCamphuijsen]

I believe this should be part of #64