Work towards #56: Splitting up the README

I did the job and splitted up the readme, hopefully everything was
splitted correctly...

thread-pool/index.md

@@ -0,0 +1,19 @@
+---
+layout: pattern
+title: Thread Pool
+folder: thread-pool
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** It is often the case that tasks to be executed are short-lived and
+the number of tasks is large. Creating a new thread for each task would make
+the system spend more time creating and destroying the threads than executing
+the actual tasks. Thread Pool solves this problem by reusing existing threads
+and eliminating the latency of creating new threads.
+
+![alt text](./etc/thread-pool.png "Thread Pool")
+
+**Applicability:** Use the Thread Pool pattern when
+
+* you have a large number of short-lived tasks to be executed in parallel