Add TFE Backwards Compatibility contrib documentation

[sebasslash]

Description

Add docs related to TFE backwards compatibility.

Fixes #996

Remember to:

• Update the Change Log
• Update the Documentation

Testing plan

1. Describe how to replicate
2. the conditions
3. under which your code performs its purpose,
4. including example Terraform configs where necessary.

External links

Include any links here that might be helpful for people reviewing your PR. If there are none, feel free to delete this section.

• go-tfe documentation
• API documentation
• Related PR

Output from acceptance tests

Please run applicable acceptance tests locally and include the output here. See testing.md to learn how to run acceptance tests.

If you are an external contributor, your contribution(s) will first be reviewed before running them against the project's CI pipeline.

$ TESTARGS="-run TestAccTFEWorkspace" make testacc

...

[rberecka]

Documentation was clear and helpful!!

[sebasslash]

This PR closes #996

[brandonc]

I hate to continue to push back on this but I still think we could get away indefinitely with not adding this type of condition to this repo. Why not just if tmAccess.ProjectAccess != nil { ... if tmAccess.WorkspaceAccess != nil { ? Not only does it look better, but it doesn't create any protocol dependency. 🤷

[sebasslash]

Yeah implicit checks are probably the simplest option here (and maybe I should call it out as the recommended approach). I like this kind of check for instances where nil checking a field isn't sufficient. It also serves as a historical record that's easier to determine "What version of TFE supports this" w/o having to dig through commit history and retrace the steps.

[brandonc]

Suggested change

	"settings": tmAccess.ProjectAccess.ProjectSettingsPermission,
	"teams": tmAccess.ProjectAccess.ProjectTeamsPermission,
	}}
	"settings": tmAccess.ProjectAccess.ProjectSettingsPermission,
	"teams": tmAccess.ProjectAccess.ProjectTeamsPermission,
	}}