improve readme with information on how to setup the environment before running Allwmake
When I first compiled OpenFOAM, I edited the config file for PETSc and have FOAM find the library. However, such information or any other possible way to customize the environment before compiling petsc4FOAM is missing in the README file. Alternatives to set up the environment before calling Allwmake would be useful for new users. For example, is it possible to switch PETSc installation (say, from debug to optimized build) and just run Allwmake again? If yes, what are the steps?