Document Procedure for Migrating psi4 modules to pasture

[amjames]

We should probably have a CONTRIBUTING.md in this repo that gives a simple list of things that should be done when moving a new module to pasture to make it easy for others to do this in the future. I have put a draft of that list below.

Required steps for moving a module to pasture

• Two PR's should be opened one here and on in psi4/psi4

In this repo the PR should include

• The module code
• Appropriate changes to the top-level CMakeLists.txt
• Modifications to the CMakeLists.txt within the module source, use ccsort/transqt2 as an example
• Run-time wrapper to go in addins.py that can be injected into proc.py

In psi4/psi4 PR

• Update PastureNotFoundError to have information for appropriate options and the modules required from pasture to use them
• Try-Except to import functions from addins.py within proc.py. If the appropriate option is specified, failure to import pasture should result in the PastureNotFoundError mentioned above.