Add "How it works" section

README.md

@@ -6,14 +6,35 @@ libpopcnt
 
 ```libpopcnt.h``` is a header only C/C++ library for counting the
 number of 1 bits (bit population count) in an array as quickly as
-possible using specialized CPU instructions e.g. POPCNT, AVX2.
+possible using specialized CPU instructions e.g.
+[POPCNT](https://en.wikipedia.org/wiki/SSE4#POPCNT_and_LZCNT),
+[AVX2](https://en.wikipedia.org/wiki/Advanced_Vector_Extensions).
 ```libpopcnt.h``` has been tested successfully using the GCC,
 Clang and MSVC compilers.
 
 The algorithms used in ```libpopcnt.h``` are described in the paper
 [Faster Population Counts using AVX2 Instructions](https://arxiv.org/abs/1611.07612)
 by Daniel Lemire, Nathan Kurz and Wojciech Mula (23 Nov 2016).
 
+How it works
+============
+```libpopcnt.h``` uses a combination of 3 different bit population
+count algorithms based on the CPU architecture and the input array
+size:
+
+* For array sizes < 1 kilobyte an unrolled ```POPCNT``` algorithm
+is used.
+* For array sizes ≥ 1 kilobyte an ```AVX2``` algorithm is used if
+the CPU supports AVX2.
+* For CPUs without ```POPCNT``` instruction a portable 
+integer popcount algorithm is used.
+
+The GitHub repository
+[WojciechMula/sse-popcount](https://github.com/WojciechMula/sse-popcount/tree/master/results)
+contains extensive benchmarks for the 3 algorithms used in
+```libpopcnt.h```. The algorithm are named
+```builtin-popcnt-unrolled```, ```avx2-harley-seal```, ```harley-seal```.
+
 C/C++ API
 =========
 ```C++
@@ -28,13 +49,13 @@ uint64_t popcnt(const void* data, uint64_t size);
 uint64_t popcnt_u64(uint64_t x);
 ```
 
-How to use it
-=============
+How to compile
+==============
 At compile time you need to specify if your compiler supports the
 ```POPCNT``` & ```AVX2``` instructions.
 
 ```bash
-# How to compile on x86 & x86_64 CPUs
+# How to compile on x86_64 CPUs
 gcc -mpopcnt -DHAVE_POPCNT -mavx2 -DHAVE_AVX2 program.c
 
 # How to compile using Microsoft Visual C++