Add Example For github_team_repository (#676)

* Add Example For `github_team_repository`

also documents a limitation with `for_each` in this scenario

* fixup! add newline

examples/repository_team/README.md

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+# Repository Team Example
+
+This demos populating a repository with a team. 
+
+This example will create a repositories in the specified `owner` organization. See https://www.terraform.io/docs/providers/github/index.html for details on configuring [`providers.tf`](./providers.tf) accordingly.
+
+Alternatively, you may use variables passed via command line:
+
+```console
+export GITHUB_OWNER=
+export GITHUB_TOKEN=
+```
+
+```console
+terraform apply \
+  -var "owner=${GITHUB_OWNER}" \
+  -var "github_token=${GITHUB_TOKEN}"
+```

examples/repository_team/main.tf

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+# NOTE: There are errors when using the following syntax. See links for details.
+# /cc https://github.com/hashicorp/terraform/issues/23529
+# /cc https://github.com/hashicorp/terraform/issues/4149
+# /cc https://github.com/integrations/terraform-provider-github/issues/500
+# 
+# data "github_team" "writers" {
+#   slug = "writer-team"
+# }
+# 
+# resource "github_team_repository" "writers" {
+#   for_each   = data.github_team.writers.id
+#   # or for multiple teams something like:
+#   # for_each   = { for obj in [data.github_team.writers] : obj.id => obj.id }
+#   team_id    = each.value
+#   repository = "repo"
+#   permission = "push"
+# }
+
+data "github_team" "writers" {
+  slug = "writers"
+}
+
+data "github_team" "editors" {
+  slug = "editors"
+}
+
+locals {
+  teams = [data.github_team.writers.id, data.github_team.editors.id]
+}
+
+resource "github_repository" "writers" {
+  name = "writers"
+  auto_init = true
+}
+
+resource "github_team_repository" "writers" {
+  count = length(local.teams)
+  team_id = local.teams[count.index]
+  repository = github_repository.writers.name 
+  permission = "push"
+}

examples/repository_team/providers.tf

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+provider "github" {
+  owner = var.owner
+  token = var.github_token
+}

examples/repository_team/variables.tf

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+variable "owner" {
+  description = "GitHub owner used to configure the provider"
+  type        = string
+}
+
+variable "github_token" {
+  description = "GitHub access token used to configure the provider"
+  type        = string
+}