Added league/pipeline.

Added league/pipeline to the Architectural section.

README.md

 * [Functional PHP](https://github.com/lstrojny/functional-php) - A functional programming library.
 * [Lib Accessor](https://github.com/phine/lib-accessor) - A library for simplifying accessors.
 * [Iter](https://github.com/nikic/iter) - A library that provides iteration primitives using generators.
+* [Pipeline](https://github.com/thephpleague/pipeline) - A pipeline pattern implementation.
 
 ## Debugging and Profiling
 *Libraries and tools for debugging and profiling code.*