Created Logging and error handling (markdown)

Logging-and-error-handling.md

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+One unfortunate thing when using some third-party libraries is that they usually pollute the console with their own logging mechanisms. CoreStore provides it's own default logging class, but you can plug-in your own favorite logger by implementing the `CoreStoreLogger` protocol.
+```swift
+final class MyLogger: CoreStoreLogger {
+    func log(#level: LogLevel, message: String, fileName: StaticString, lineNumber: Int, functionName: StaticString) {
+        // pass to your logger
+    }
+    
+    func handleError(#error: NSError, message: String, fileName: StaticString, lineNumber: Int, functionName: StaticString) {
+        // pass to your logger
+    }
+    
+    func assert(@autoclosure condition: () -> Bool, message: String, fileName: StaticString, lineNumber: Int, functionName: StaticString) {
+        // pass to your logger
+    }
+}
+```
+Then pass an instance of this class to `CoreStore`:
+```swift
+CoreStore.logger = MyLogger()
+```
+Doing so channels all logging calls to your logger.
+
+Note that to keep the call stack information intact, all calls to these methods are not thread-managed. Thus you have to make sure that your logger is thread-safe or you may otherwise have to dispatch your logging implementation to a serial queue.
+
+## <a id="observing"></a>Observing changes and notifications
+CoreStore provides type-safe wrappers for observing managed objects:
+
+- `ManagedObjectController`: use to observe changes to a single `NSManagedObject` instance (instead of Key-Value Observing)
+- `ManagedObjectListController`: use to observe changes to a list of `NSManagedObject` instances (instead of `NSFetchedResultsController`)
+
+
+## Contents
+- [[Architecture]]
+- [[Setting up]]
+- [[Saving and processing transactions]]
+- [[Fetching and querying]]
+- [[Logging and error handling]]
+- [[Observing changes and notifications]]