Add some documentation for memberships

Author: lucasmichot

doc/currentuser/memberships.md

@@ -0,0 +1,36 @@
+## Current user / Memberships API
+[Back to the navigation](../README.md)
+
+Wraps [GitHub Issue Comments API](https://developer.github.com/v3/orgs/members/#get-your-organization-membership).
+
+### List your memberships
+
+> Requires [authentication](../security.md).
+
+```php
+$memberships = $client->user()->memberships()->all();
+```
+
+Returns an array of your memberships in all organizations you are part of.
+
+### Show an organization membership
+
+> Requires [authentication](../security.md).
+
+```php
+$membership = $client->user()->memberships()->organization('KnpLabs');
+```
+* `KnpLabs` : the organization
+
+Returns an array of one membership in a specific organization.
+
+### Update an organization membership
+
+> Requires [authentication](../security.md).
+
+```php
+$membership = $client->user()->memberships()->edit('KnpLabs');
+```
+* `KnpLabs` : the organization
+
+Update your membership to an organization. The only possible action is to activate your membership.