Add TFE backwards compatibility docs

This document outlines how to handle unsupported TFE versions, TFE only behavior,
and updating the documentation to reflect supported TFE versions.

Author: sebasslash

docs/backwards-compatibility.md

@@ -0,0 +1,68 @@
+# Maintaining Backwards Compatibility (Terraform Enterprise)
+
+## Terraform Enterprise Version Checking
+
+**Note:** If you are introducing a new resource that is only available at a certain TFE release, you do not need to perform any sort of checks.
+
+### Implied Check (Recommended Approach)
+
+The simplest solution when a particular attribute is not supported by a given TFE release is a `nil` check. These checks are TFE release agnostic and assume that if a field was not returned by the API, the TFE release does not support it. They are particularly important for fields that are structs, where attempting to dereference a nil pointer causes a panic.
+
+You can use this implied checks when:
+
+- The field is a pointer
+- When a `null` value is ignored by the API or by go-tfe (see if the struct tag has `omitempty`)
+
+**Example:**
+
+```go
+if tmAccess.ProjectAccess != nil {
+    projectAccess := []map[string]interface{}{{
+    		"settings": tmAccess.ProjectAccess.ProjectSettingsPermission,
+    		"teams":    tmAccess.ProjectAccess.ProjectTeamsPermission,
+    	}}
+    // Write project access to state
+    err := d.Set("project_access", projectAccess)
+}
+```
+
+### Explicit Enterprise Checks
+
+If a resource or attribute is **only** available in Terraform Enterprise, use the go-tfe helper [IsEnterprise()](https://pkg.go.dev/github.com/hashicorp/go-tfe#Client.IsEnterprise) to ensure the client is configured against a TFE instance. This check is derived from the `TFP-AppName` header that Terraform Cloud emits, of which if not present, indicates a Terraform Enterprise installation.
+
+```go
+config := meta.(ConfiguredClient)
+
+if config.Client.IsEnterprise() {
+    // do something with TFE only behavior
+}
+```
+
+### Documentation
+
+It is important to communicate with practitioners which resources and fields are supported for a particular TFE release.
+
+For a new resource, add the minimum release required to the top level documentation.
+
+**Example:**
+
+```md
+# my_new_resource
+
+Provides a my new resource.
+
+~> **NOTE:** Using this resource requires using the provider with Terraform Cloud or an instance of Terraform Enterprise at least as recent as v202302-1.
+```
+
+
+If an attribute has a TFE release constraint, add a second sentence to the attribute's description:
+
+```md
+## Argument Reference
+
+The following arguments are supported:
+
+* `foo` - (Required) Foo is bar.
+* `bar` - (Optional) Bar is foo.
+* `foobar` - (Optional) Foobar is barfoo. This attribute requires Terraform Cloud or an instance of Terraform Enterprise at least as recent as `v202302-1`.
+```

mkdocs.yml

@@ -15,6 +15,7 @@ nav:
           - Testing: testing.md
           - Introducing Beta Features: beta.md
           - Debugging: debugging.md
+          - Backwards Compatibility: backwards-compatibility.md
       - Acceptance Tests: writing-acceptance-tests.md
       - Changelog Process: changelog-process.md