Move background to a separate motivation document

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

MOTIVATION.md

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+Motivation
+----------
+
+> “Why an event loop, why not use threads?”
+
+With the advent of light-weight processes (threads) programmers these
+days have a [golden hammer](http://c2.com/cgi/wiki?GoldenHammer) they
+often swing without consideration.  Event loops and non-blocking I/O is
+often a far easier approach, as well as less error prone.
+
+The purpose of many applications is, with a little logic sprinkled on
+top, to act on network packets entering an interface, timeouts expiring,
+mouse clicks, or other types of events.  Such applications are often
+very well suited to use an event loop.
+
+Applications that need to churn massively parallel algorithms are more
+suitable for running multiple (independent) threads on several CPU
+cores.  However, threaded applications must deal with the side effects
+of concurrency, like race conditions, deadlocks, live locks, etc.
+Writing error free threaded applications is hard, debugging them can be
+even harder.
+
+Sometimes the combination of multiple threads *and* an event loop per
+thread can be the best approach, but each application of course needs to
+be broken down individually to find the most optimal approach.  Do keep
+in mind, however, that not all systems your application will run on have
+multiple CPU cores -- some small embedded systems still use a single CPU
+core, even though they run Linux, with multiple threads a program may
+actually run slower!  Always profile your program, and if possible, test
+it on different architectures.