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.

* website/contributing: update contribution documentation

This PR seeks to remove outdated and incorrect information. There is
still work to be done updating the information that's left; this is
merely the first step.

* docs: install tools before building in docker environment

* Update README.md
Co-Authored-By: Kristin Laemmert <mildwonkey@users.noreply.github.com>

The "terraform" provider was previously split out into its own repository,
but that turned out to be a mistake due to how tightly it depends on
aspects of Terraform Core.

Here we prepare to bring it back into the core repository by reorganizing
the directory layout to conform with what's expected there.

This improves the testing and other tooling and thus allows us to fix
some annoyances. (These fixes will follow in subsequent commits)

* Update README with link to "Getting started" guide

* spellin

spell "infrastructure" correctly

This now uses the canonical location for the logo image on the main Terraform website.

(The image isn't actually there at the time of this commit, but will be published here as
part of updating the website for the 0.10.0 release.)

The intent here is just to introduce some initial docs on our recommended
way to develop plugins in the same GOPATH as Terraform itself. The
documentation in this area needs some more fundamental rework as it is
rather outdated and mis-organized, but that's outside the scope of what
this change is trying to achieve.

This commit adds a link to the GitHub organization which contains the
formerly-built-in providers, and modifies the `plugin-dev` target
documentation to use a provider which is unlikely to be moved out of the
core repository.

$ make core-dev
make: *** No rule to make target `core-dev'.  Stop.

This was removed in #15258

Use HTTPS in terraform website url.

IRC has been deprecated in favor of Gitter, this links directly to the Gitter chat room
Signed-off-by: Dave Walter <dwalter@pivotal.io>