Added sagan pages; fixes gh-559

docs/src/main/asciidoc/sagan-index.adoc

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+Spring Cloud Commons delivers features as two libraries: Spring Cloud Context and Spring Cloud Commons. Spring Cloud Context provides utilities and special services for the ApplicationContext of a Spring Cloud application (bootstrap context, encryption, refresh scope and environment endpoints). Spring Cloud Commons is a set of abstractions and common classes used in different Spring Cloud implementations (eg. Spring Cloud Netflix vs. Spring Cloud Consul).
+
+## Features
+
+### Spring Cloud Context features:
+
+* Bootstrap Context
+* `TextEncryptor` beans
+* Refresh Scope
+* Spring Boot Actuator endpoints for manipulating the `Environment`
+
+### Spring Cloud Commons features:
+
+* `DiscoveryClient` interface
+* `ServiceRegistry` interface
+* Instrumentation for `RestTemplate` to resolve hostnames using `DiscoveryClient`