all index.md files renamed to README.md for more compatibility with github

thread-pool/README.md

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+---
+layout: pattern
+title: Thread Pool
+folder: thread-pool
+permalink: /patterns/thread-pool/
+categories: Concurrency
+tags:
+ - Java
+ - Difficulty-Intermediate
+ - Performance
+---
+
+## 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