Add resource_project_branch_permissions (#47)

Resolves gavinbunney/terraform-provider-bitbucketserver#17

docs/resources/bitbucketserver_project_branch_permissions.md

@@ -0,0 +1,33 @@
+# Resource: bitbucketserver_project_branch_permissions
+
+Provides the ability to apply project ref restrictions to enforce branch permissions. A restriction means preventing writes on the specified branch(es) by all except a set of users and/or groups, or preventing specific operations such as branch deletion.
+
+## Example Usage
+
+```hcl
+resource "bitbucketserver_project_branch_permissions" "pr_only" {
+  project          = "MYPROJ"
+  repository       = "repo"
+  ref_pattern      = "refs/heads/master"
+  type             = "pull-request-only"
+}
+
+resource "bitbucketserver_project_branch_permissions" "no_deletes" {
+  project     = "MYPROJ"
+  repository  = "repo"
+  ref_pattern = "heads/**/master"
+  type        = "no-deletes"
+  exception_users  = ["admin"]
+  exception_groups = ["group_1", "group_2"]
+}
+```
+
+## Argument Reference
+
+* `project` - Required. Project Key that contains target repository.
+* `repository` - Required. Repository slug of target repository.
+* `ref_pattern` - Required. A wildcard pattern that may match multiple branches. You must specify a valid [Branch Permission Pattern](https://confluence.atlassian.com/bitbucketserver/branch-permission-patterns-776639814.html).
+* `type` - Required. Type of the restriction. Must be one of `pull-request-only`, `fast-forward-only`, `no-deletes`, `read-only`.
+* `exception_users` - Optional. List of usernames to whom restrictions do not apply.
+* `exception_groups` - Optional. List of group names to which restrictions do not apply.
+* `exception_access_keys` - Optional. List of access keys IDs to which restrictions do not apply.