Updates README.md with rationale.

Author: kzaher

README.md

@@ -56,6 +56,12 @@ Observable.just([SectionModel(model: "title", items: [1, 2, 3])])
 ```
 ![RxDataSources example app](https://raw.githubusercontent.com/kzaher/rxswiftcontent/rxdatasources/RxDataSources.gif)
 
+## Why was custom diff algorithm used?
+
+- All dynamic programming diff solutions (longest common subsequence problem) have worst quardratic time complexity O(n^2). If you are interested in exploring those solutions, we recommend [Diff](https://github.com/wokalski/Diff.swift#diffswift).
+- It's even more problematic to reconcile tree edit distance algorithms with `UI{Table,Collection}View` animated section updates.
+- In all practial cases each section item is uniquely identified and that can be used to optimize the algorithm.
+
 ## How
 Given the following custom data structure:
 ```swift