Work towards #56: Splitting up the README

I did the job and splitted up the readme, hopefully everything was
splitted correctly...

service-layer/index.md

@@ -0,0 +1,21 @@
+---
+layout: pattern
+title: Service Layer
+folder: service-layer
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Service Layer is an abstraction over domain logic. Typically
+applications require multiple kinds of interfaces to the data they store and
+logic they implement: data loaders, user interfaces, integration gateways, and
+others. Despite their different purposes, these interfaces often need common
+interactions with the application to access and manipulate its data and invoke
+its business logic. The Service Layer fulfills this role.
+
+![alt text](./etc/service-layer.png "Service Layer")
+
+**Applicability:** Use the Service Layer pattern when
+
+* you want to encapsulate domain logic under API
+* you need to implement multiple interfaces with common logic and data