Our documentation for how to contribute was in quite a state of disrepair,
with some documents still describing things as they were before moving
providers into separate repositories, others making assumptions about
Go development that are no longer true in modules mode, and so forth.

This is an attempt at a reset to a good state that should work with the
codebase as it currently stands, and should hopefully serve as a basis
for iterative improvement from here.

These new instructions lean primarily on standard Go toolchain usage and
instruct using the Makefile only for some Terraform-specific situations
that the Go toolchain does not automatically handle. The idea here is that
this direct usage of primary commands in the Go toolchain is less likely
to be broken by changes in future Go releases, and should be immediately
familiar to anyone who has experience with Go development.