README: provide a bit more information on why SIMDe is useful
SIMDe provides fast, portable implementations of SIMD intrinsics on
hardware which doesn't natively support them, such as calling SSE
-functions on ARM.
+functions on ARM. There is no performance penalty if the hardware
+supports the native implementation (*e.g.*, SSE/AVX runs at full speed
+on x86, NEON on ARM, *etc.*).
+
+This makes porting code to other architectures much easier in a few
+key ways:
+
+First, instead of forcing you to rewrite everything for each
+architecture, SIMDe lets you get a port up and running almost
+effortlessly. You can then start working on switching the most
+performance-critical sections to native intrinsics, improving
+performance gradually. SIMDe lets (for example) SSE/AVX and NEON code
+exist side-by-side, in the same implementation.
+
+Second, SIMDe makes it easier to write code targeting ISA extensions
+you don't have convenient access to. You can run NEON code on your
+x86 machine *without an emulator*. Obviously you'll eventually want
+to test on the actual hardware you're targeting, but for most
+development SIMDe can provide a much easier path.
+
+SIMDe takes a very different approach from most other SIMD abstraction
+layers in that it aims to expose the entire functionality of the
+underlying instruction set. Instead of limiting functionality to a
+lowest common denominator, SIMDe tries to minimize the amount of
+effort required to port while still allowing you the space to optimize as
+needed.
The current focus is on writing complete portable implementations,
though a large number of functions already have accelerated
