Add README for OAuth2 client sample

spring-boot-samples/spring-boot-sample-oauth2-client/README.adoc

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+= Spring Boot Sample OAuth2 Client
+
+== Register Github OAuth2 application
+To run the sample, you need to link:https://github.com/settings/applications/new[register an OAuth application on Github].
+While registering your application, ensure the Authorization callback URL is set to http://localhost:8080/oauth2/authorize/code/github.
+After completing the registration, you will have a new OAuth Application with a Client ID and Client Secret.
+
+== Configuring application.yml
+Once the OAuth application is registered with GitHub, you need to configure the sample application to use this OAuth application (client).
+Edit the link:src/main/resources/application.yml[application.yml] and replace ${APP-CLIENT-ID} and ${APP-CLIENT-SECRET} with the OAuth client credentials created in the previous section.
+
+The sample can now be run and you can login with your Github user credentials.