[docs] update README

Author: albarrentine

README.md

@@ -1,2 +1,4 @@
 # utf_transcoding
-Transcoding UTF-8/Latin-1/UTF-16/UTF-32 with SIMDe AVX2 functions
+Transcoding UTF-8/Latin-1/UTF-16/UTF-32 with SIMDe AVX2 functions.
+
+This is a port of [simdutf](https://github.com/simdutf/simdutf) to pure C using cross-platform AVX2 instructions with [simde](https://github.com/goodcleanfun/simde_avx2).