Informing the end-users about best practices for packaging Haskell code with(out) non-portable C flags.

Some package authors will want to enable C flags for their FFI C code in order to get good performance (or avoid terrible performance) by default for their users. However some of these flags are not portable across compilers and versions.

`-march=native` for instance, gives a solid performance boost, but is unsuitable for distributing binaries. We should probably encourage the pattern of having a `cabal.project` for local development and a `cabal.release.project` file that enables it.

This information would be located in the user guide's *How to package Haskell code* section.