Add SDK Documentation

Author: jessjenkins

README.md

@@ -4,7 +4,8 @@ API for managing access control permissions for Digital Publishing API resources
 
 ### Getting started
 
-* Load inital roles into your local mongodb using the `import-script` utility in the import-script folder. Follow the steps in the [README](./import-script/README.md).
+* Load inital roles into your local mongodb using the `import-script` utility in the import-script folder. Follow the
+  steps in the [README](./import-script/README.md).
 * Run `make debug`
 
 ### Dependencies
@@ -34,6 +35,10 @@ API for managing access control permissions for Digital Publishing API resources
 | DEFAULT_OFFSET                 | 0                                                   | Default offset for pagination                                                                                       |
 | DEFAULT_MAXIMUM_LIMIT          | 1000                                                | Default maximum limit for pagination                                                                                |
 
+### SDK Package
+
+[An SDK for the API is available as a subpackage in `/sdk`](sdk/README.md)
+
 ### Contributing
 
 See [CONTRIBUTING](CONTRIBUTING.md) for details.

sdk/README.md

@@ -0,0 +1,45 @@
+dp-permissions-api SDK
+======================
+
+## Overview
+
+This API contains a client with convenience functions for interacting with the permissions api from other applications.
+It also contains reusable structs representing models used as payloads in API endpoints.
+
+## Example use of the API SDK
+
+```go
+package main
+
+import (
+	"context"
+	"github.com/ONSdigital/dp-permissions-api/sdk"
+)
+
+func main() {
+	apiClient := sdk.NewClient("http://localhost:25400")
+
+	permissionsBundle, err := c.cache.GetPermissionsBundle(context.Backgroud)
+}
+```
+
+## Alternative Client instantiation
+
+In the unlikely event that there is a need to use non-default initialisation, it is possible to obtain a new client with
+customised options and/or underlying http client.
+
+### With Options
+
+NB. There are currently no defined options available, this is included for future expansion.
+
+```go
+apiClient := sdk.NewClientWithOptions("http://localhost:25400", sdk.Options{})
+```
+
+### With Options and a custom http client
+
+```go
+import dphttp "github.com/ONSdigital/dp-net/v2/http"
+
+apiClient := sdk.NewClientWithClienter("http://localhost:25400", dphttp.NewClient(), sdk.Options{})
+```