Adds Generator readme.

Preprocessor/README.md

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+## RxSwift KISS code generator.
+
+RxSwift code is partially machine generated. 
+It converts *.tt files into generated *.swift files.
+
+There are multiple reasons why that is the case.
+
+* Performance
+* Consistency
+* Removes burden from developers (me :)
+* It's fun
+
+To see how it in action, take a look at 
+
+CombineLatest+arity.tt
+CombineLatest+arity.swift
+
+```
+//
+//  CombineLatest.tt.swift
+//  RxSwift
+//
+//  Created by Krunoslav Zaher on 4/22/15.
+//  Copyright (c) 2015 Krunoslav Zaher. All rights reserved.
+//
+
+import Foundation
+
+<% for i in 2 ... 10 { %>
+
+// <%= i %>
+
+public func combineLatestOrDie<<%= ", ".join(map(1...i) { "E\($0)" }) %>, R>
+    (<%= ", ".join(map(1...i) { "source\($0): Observable<E\($0)>" }) %>, resultSelector: (<%= ", ".join(map(1...i) { "E\($0)" }) %>) -> Result<R>)
+        -> Observable<R> {
+    return CombineLatest<%= i %>(
+        <%= ", ".join(map(1...i) { "source\($0): source\($0)" }) %>,
+        resultSelector: resultSelector
+    )
+}
+
+public func combineLatest<<%= ", ".join(map(1...i) { "E\($0)" }) %>, R>
+    (<%= ", ".join(map(1...i) { "source\($0): Observable<E\($0)>" }) %>, resultSelector: (<%= ", ".join(map(1...i) { "E\($0)" }) %>) -> R)
+        -> Observable<R> {
+    return CombineLatest<%= i %>(
+        <%= ", ".join(map(1...i) { "source\($0): source\($0)" }) %>,
+        resultSelector: { success(resultSelector(<%= ", ".join(map(0..<i) { "$\($0)" }) %>)) }
+    )
+}
+```
+
+to 
+
+```
+// This file is autogenerated.
+// Take a look at `Preprocessor` target in RxSwift project 
+//
+//  CombineLatest.tt.swift
+//  RxSwift
+//
+//  Created by Krunoslav Zaher on 4/22/15.
+//  Copyright (c) 2015 Krunoslav Zaher. All rights reserved.
+//
+
+import Foundation
+
+
+
+// 2
+
+public func combineLatestOrDie<E1, E2, R>
+    (source1: Observable<E1>, source2: Observable<E2>, resultSelector: (E1, E2) -> Result<R>)
+        -> Observable<R> {
+    return CombineLatest2(
+        source1: source1, source2: source2,
+        resultSelector: resultSelector
+    )
+}
+
+public func combineLatest<E1, E2, R>
+    (source1: Observable<E1>, source2: Observable<E2>, resultSelector: (E1, E2) -> R)
+        -> Observable<R> {
+    return CombineLatest2(
+        source1: source1, source2: source2,
+        resultSelector: { success(resultSelector($0, $1)) }
+    )
+}
+
+```
+
+
+It's pretty generic, I'm thinking of extracting it as a separate project.