Add OpenID test documentation

* Write a paragraph that informs contributors about the openID test
accounts that are available and provide instructions on how to
enable them in development (#1315) in CONTRIBUTING.md

* Add missing environment variable for openID logins via openSUSE
(OSEM_SUSE_KEY, OSEM_SUSE_SECRET) in dotenv.example

CONTRIBUTING.md

@@ -169,6 +169,18 @@ config.ichain_force_test_username = "testuser"
 # set email of 'testuser'
 config.ichain_force_test_attributes = {:email => "testuser@example.com"}
 ```
+
+### Using OpenID in developement
+OSEM supports [OpenID](https://openid.net/) logins via [OmniAuth](https://github.com/omniauth/omniauth) and related provider specific gems. OmniAuth provides the ablity to define per-provider mock accounts for testing. The supported providers are Facebook, Google, openSUSE and GitHub. If you want to use the OSEM provided mock accounts you need to set the appropriate `OSEM_PROVIDER_KEY` and `OSEM_PROVIDER_SECRET` environment variables to a non empty string in the `.env` file.
+
+e.g.
+```
+OSEM_GITHUB_KEY='sample'
+OSEM_GITHUB_SECRET='sample'
+```
+
+If you don't already have a `.env` file you can use the `dotenv.example` as a template.
+
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