Add IterTools PHP to Architectural section. (#1217)

README.md

 * [Finite](https://yohan.giarel.li/Finite/) - A simple PHP finite state machine.
 * [Functional PHP](https://github.com/lstrojny/functional-php) - A functional programming library.
 * [Iter](https://github.com/nikic/iter) - A library that provides iteration primitives using generators.
+* [IterTools PHP](https://github.com/markrogoyski/itertools-php) - A library that provides functionality for working with iterable entities (similar to itertools library in Python).
 * [Patchwork](https://patchwork2.org/) - A library for redefining userland functions.
 * [Pipeline](https://github.com/thephpleague/pipeline) - A pipeline pattern implementation.
 * [Porter](https://github.com/ScriptFUSION/Porter) - Data import abstraction library for consuming Web APIs and other data sources.